I've been using a 8MB RAM expansion card in my A1200 for years. I go online, use plenty of applications, and played every WHDLoad game out there. There is no need for an accelerator if you don't plan on anything more than this.
Amigas can address up to 4MB SRAM, but 4MB cards are hard to come by compared to 2MB cards.
There are no brand new A1200 RAM expansions.
There are brand new A600 ChipRAM expansions in the form of the A603.
There will be 8MB FastRAM expansions for the A600 in coming months in the form of the A608 from Zetr0 (based on the Russian design).
There are some expensive, new-old-stock accelerators available for the A1200.
SRAM will speed up an A600 a small amount.
SRAM will slow down an A1200 a small amount.
SRAM will not affect the speed of regular games, but it will allow you to play them.
Here are some SysInfo results to help you make up your own mind:
A600 (2MB Chip, No FastRAM)
526 Dhrystones
0.54 MIPS
A600 (2MB Chip, 2MB PCMCIA SRAM)
699 Dhrystones
0.72 MIPS
A1200 (2MB Chip, No FastRAM)
1092 Dhrystones
1.13 MIPS
A1200 (2MB Chip, 2MB PCMCIA SRAM)
840 Dhrystones
0.87 MIPS
A1200 (2MB Chip, 8MB FastRAM)
2819 Dhrystones
2.94 MIPS