gtx285! The gtx 295 is essentially gtx275's in sli and if you get sli gtx285's then your just one off getting tri sli gtx285 which is faster then sli gtx295's
If you want the easiest, least hassle upgrade path, go with the best single gpu you can afford. This is the path I am choosing. I went with a mobo that can be upgraded with a Phenom II, overclocks well, has 1 PCI-E x16 2.0 slot so I can upgrade to at least an HD 4890 without overdoing my 550W PSU, and I can just keep doing this. Eventually I may need a new power supply, which will be a pain to upgrade... but by that time it will probably have been 2 years or so and I'll just build a whole new system on an i7 by then
Also, SLI or Crossfire won't guaranty a specific amount of increase for all applications. Games, for instance, vary on how they accept 2 GPUs. Some are optimized and some aren't.
Why would I skip it? It will probably be the perfect option for my price range by then. I should have gone with Core 2 Duo or Quad or Phenom II's this time around, but I went for the dirt cheap X2.