Hi, I did exactly what you are planning to do recently. I had a PLP setup with a Dell 3007 and upgraded by adding two Dell 3008. I also switched from SoftTH to Eyefinity with the 5970. The new setup is impressive when everthing works, but it is still hit and miss for me. Games like Call of Duty: WaW, GRID and the Source titles run totally smooth at high settings, but I haven't had luck others like CoD4 or the Crysis games yet, which I got to run better with SoftTH and a GTX285. I must say it is still early days and I have a lot more experimenting to do.
The 1Gb framebuffer also worries me a bit. FarCry 2, for example, runs smooth as silk but occasionally freezes for a split second when I run or drive. Lowering the detail settings doesn't seem to help. This never happened with my GTX285 with 2Gb VRAM, although it is not as fast. For this reason I think that crossfiring a 5970 would be a suboptimal solution, and I'll wait for cards with more VRAM before I would upgrade again.
Overall I'm very happy with this setup and I certainly recommend it, as you can expect the drivers to improve.
I would think the 2GB frame buffer explains why Crysis ran better on your GTX 285 at that res, since I've heard the game is fairly tough on the graphics memory.
When you say PLP setup, I assume you're talking 4960X1600 with a 30 inch and two 20s? What did you have to do to get Crysis to run on that with a GTX 285?
Although I'm surprised a GTX 285 had enough power to run the game well on a PLP setup