A graphics card upgrade would probably be plenty to keep that system running smooth for quite a while.
IMO, you have a few options. Buy a GTX 260 216 core for around $180 (a bargain), buy a GTX 275 for around $230 (also a good buy) or wait for the GT300 from nvidia. The specs are more than twice the GTX 285's in almost every way. There's a good chance it'll be available before October. It should beat pretty much everything available now, other than quad SLI or quad crossfire setups... and it'll likely beat those in some areas, without any multi-gpu headaches.
If you have a bit of extra money available though, by all means, upgrade now. You can always sell and grab a GT300 later, depending on how crazy you are when it comes to upgrading.
Also, as methim said, you don't need PCI-E 2.0, it mainly just helps with SLI or Crossfire setups. I don't really think the GTX 285 is the best buy right now though. Its the best single-card available, sure, but I don't think its a big enough leap over the 275 to make it worth the extra $100.