I'd recommend waiting for driver maturity, stability, and all the bugs to be worked out. Nvidia doesn't have a driver yet that will support SLI under Vista so you'll be crippled with one card until then. Also, Vista strips DirectSound and Directsound 3d since the native audio API is OpenAL. Everything gets converted to stereo, so your sound is crippled too. Supposedly Creative is going to be releasing a driver to convert everything back to an emulated DirectSound and DS3d for older games that don't support OpenAL, but that only helps if you have a Creative sound card.
Now, that's my official recommendation. What would I do personally if I were in your situation? I'd probably get the 2 gig Ram upgrade and immediately upgrade to Vista and wallow in all the bugs because I'm way too curious for my own good and don't mind letting it get me into trouble at times.