Still it was a bit strange that you come up recommending something totally overpowered (8400 GS -> 9800 GX2 ?) also being nowhere in the 200$ budget (plus the fact that the 9800 X2 isn't produced anymore and such being hardly available)...
It also isn't surprising that if SLI works the GX2 will be faster than the GXT250 you had as that one is just a new labeled 9800 GTX+. SLI has the usual problems of not always working correct (although there are tools to help here) and not always getting a good boost for the 2nd GPU. Then there of course is the issue of micro stuttering making things not that enjoyful when the framerate starts getting below let's say 40fps (the exact value from which you'll notice differs though). That simply happens due to the AFR method currently all multi-gpu systems use for rendering (Video)...
Could you give examples (and I'm not talking about PhysX oder CUDA here)?but theres still stuff that ATI doesn't support