Accelerator-wise, everything depends on what you want to do, and how far you're willing to go. In addition to 32-bit acceleration and memory (and those things already mentioned), you can upgrade to RTG (better graphics card) and sound, and OS3.9.
A more modest expansion might be a Supra Turbo28, a 2091 SCSI and ram, and maybe stock graphics with a flicker-fixer. This is still a totally 16-bit system and may be more compatible with older 1.3 games (if that's important to you).
The 16-bit route is not necessarily cheaper than the 32-bit.
I have 3 2000s, 1 with a Supra Turbo, 1 with A2630 and DKB's memory add-on, and another with GVP's combo board. They're all fun.
I only rescued the 2000s a few years ago, the big surprise to me was the rom of the SCSI interface may also have to be upgraded in addition to the system rom.
