I think if was starting an A2000 build today it would go like this
1. Buy either of the following
Masoboshi Master Card1x Internal 50pin Fast SCSI II DMA controller
1x internal 40 pin IDE header (translates SCSI commands to IDE)
1x External 25pin SCSI connector
Also supports up to 8MB of FAST RAM (in 1Mx4 ZIP's - can be expensive)
2x devices on the IDE header (these are seen as devices 8 and 9)
7x devices on the SCSI bus (1 is taken up by the card as HOST)
External devices require active powered termination
Internal devices can be passively terminated
Speeds (4MB FAST 2MB CHIP / Stock 68000 CPU @7ish MHz)
IDE 1.46MB per second (81% CPU idle 19% in use) [ 40GB Hitachi Travelstar 2.5" 5400 RPM 9.1ms Seek ]
SCSI 1.5MB per second (88% CPU idle 12% in use) [ 9.1 GB Compaq 3.5" 10k RPM 3.4ms Seek ]
*please note the more idle the CPU the more responsive it is to taking on new tasks and or completing existing tasks
GVP Impact Series II HC8+1x 50pin internal FAST SCSI II
1x 25pin external FAST SCSI II
Also supports up to 8MB of FAST RAM (easy to obtain 1MB SIMM's)
7x Devices on the SCSI bus (1 is taken up by the card as HOST)
External devices require active powered termination
Internal devices can be passively terminated
Speeds (8MB FAST 2MB CHIP / Stock 68000 CPU @7ish MHz)
SCSI 1.6MB per second (91.9% CPU idle 8.9% in use)[ 9.1 GB Compaq 3.5GB 10k RPM 3.4ms Seek ]
*please note the more idle the CPU the more responsive it is to taking on new tasks and or completing existing tasks
actually I have a picture of this test -

Thats pretty amazing - over 1.6 Megabytes a second transfer on a stock 7Mhz 68k moto - and its only using just under 9% of the CPU time.... thats really impressive!
Now let me show you this in contrast -
Here is an A1200 equipped with an Apollo 030@40Mhz with 64MB of FAST RAM - running a 20GB Samsung Spin point (PATA 133) from the native A1200 IDE

As you can see it achieves a reasonable 1.6MB per second of data transfer - however if you notice only 0.2% of processor is available for other tasks - sadly IDE (when implemented as the native IDE on an A1200) can literally cripple a system.
What is even more of a surprise is to know that the Apollo 030@40Mhz with 64MB of FAST RAM achieves about 8.5MIP's of which nearly all (bar 0.2% - 0.017MIPS) is used up getting the data from the device / IDE bus!
When you stack that against 0.67MIPs of a stock 7MHz 68k with FAST RAM that achieves 1.67MB and only uses 8.9% of that (0.67MIPS i.e. 0.0603 MIPS) to achieve a faster speed - one can clearly see the awesome of managed (DMA Based) FAST SCSI II - in fact the the stock 68k CPU has nearly 35.84 times more processing power available compared to the 030@40 MHz equipped A1200 on native IDE ( thats 0.6097 remaining MIPS / 0.017 remaining MIPS)
So yes.... in a nut shell, you want managed (DMA based) FAST SCSI II and you will be loving your life!
Phew.... now back to the topic... (sorry about that)
after you have Hard Disk and Fast Ram I would then suggest saving your pennies for a 1MB CHIP upgrade (giving you 2MB CHIP total).
then you will have a quite a few avenues available to you - pending your target machine implementation.
Next on the agenda would be a CPU upgrade, there are a few options here - you could use the 68k socket (replacing the stock 68k moto) or use the CPU upgrade socket.
Your budget will really decide where you go, but even an A2620 would be a delight - offering 4MB (in 32bit) - this will take a 0.6MIP (stock 68k with FAST RAM) and give you about 3 MIPS with the 020 and 32bit FAST RAM
I should point out that with the A2620 (and A2630) they use the 24bit address space - so the memory on the accelerator card is removed from the memory pool - such that with 4MB on the card - it will only allow up to a further 4MB with your other adaptors for a total of 8MB of FAST RAM. It is important to know that you will have both narrow (16bit wide) and wide (32bit wide) memory pools - this will be transparent to you (and your applications) for the most part - but those applications (like say Octamed Sound Studio) that would use large samples or Imagine Raytracer with large models may experience a minor slow-dow as the CPU has to shift from 32bit to 16bit data paths and vice versa
In case you have not guessed I do own 4 A2000's in various upgrades states =)
Anyway once you have your CPU upgrade, as other have mentioned you should look at getting a scan doubler / flicker fixer. In one of my A2000's I use a Microway flicker fixer / scan doubler and this serves very well - however you would be very well served with an external
GBS8220 (rev 5 or higher *aprx £20) or my favourite the
Indivision ECS *aprx £80
I should point out that the Indivision ECS is more than just a scan doubler / flicker fixer - it also offers Hi-GFX support as well as lots of other screen modes. It even has a beta Picasso Graphics mode (800x600)
____________
So at this point you should have
Hard Disk
FAST RAM
Upgraded CPU
Upgraded Native Video
My next choice would be between upgrading the Sound or adding an RTG graphics card. If I had the Indivsion installed I would for the most part not consider RTG graphics as a priority and go straight to sound - however without that I would go for sound next and while there are a few cards none are as supported as the Toccata sound card - this gives you 16bit sound processing with multiple channels - which is pretty damn awesome if you are into writing music - if not - skip this and get a graphics card.
If you decide to get a graphics card there really is only one to get (unless you want to mortgage your house) and that is the Picasso II (or Picasso II+) these are Z2 RTG graphics cards that will really bring out the power of your Amiga and seriously enhance your Amiga experience - theres something very wholesome about playing DOOM on my A2000 alongside a 1024x768 in 16bit workbench
So, now you haveHard Disk (Mass Storage)
FAST RAM
Upgraded CPU
Upgraded Native Graphics / Video
Upgraded Sound
Upgraded Graphics (RTG)
where next?well then I would recommend some networking card - my favourite is the XSurf2... then perhaps you might like to dabble in other areas - the A2000 is a real joy - One of my A2000's is a hybrid build which includes a 1GHz P3 SBC hooked up along side the 040@33Mhz
but that is because I am crazy!