You don't have to switch to AMD to use the HD 5870. And right now, nothing beats it. I would just stick it in your current computer. No reason to spend the extra $400 that's not needed.
Also, Crossfiring 5870s is just stupid. The cost of that would be $800. The HD 5970 is essentially two 5870s Crossfired on one board for $650-700. The only difference is that the 5970 has downclocked the GPUs slightly, which can easily be undone. Also, the 5970 will only take up one slot, making it even more future proof, not that you should ever need to Crossfire 5970s...
I would wait until either DirectX 11 becomes useful or you're not running newer games at playable speeds (i.e. 30 FPS). By then, Fermi should be out, and either they'll be better cards (which isn't the indication) or they'll be some kind of competition for the 5xxx series to be cheaper. Either way, you get a better card or a cheaper price.
@MadAdmiral... he didn't mention his case by name... so I assume the worst. I did notice the 3 GPUs, but I can picture a lot of older full size cases (like a TT Armor) that definitely have the height, but still might (or might not!) be able to accommodate the length of the newer cards. Video cards pushing 13 inches weren't really considered when some previous-gen cases were designed.
i was not going to get the 5970 i was going to pick up the 5870 and my case right now with the 9800gtx's in it is a little tight enough room for the psu cables to run between the back of the cards and the drive bays with a little airflow i was looking at the raven 2 as it look to have more manageable room
although its 90* so i have no idea what space is actually usable for gpu space i will head to frys today with measuring tape in hand
@ MadAdmiral: Maximum GPU length is an EXCELLENT idea for a field in the product descriptions at NewEgg. Hell, take it one step further... if you order a case and a GPU, do a basic compatibility check for the buyer and warn them if they appear to be doing something silly.