It appears that someone wants to post his opinions of Nvidia's supposed supperior features as if they are facts and ignore certain features AMD provides that you cannot get on Nvidia cards to start another stupid AMD vs. Nvidia thread that goes on forever. Some things never change.
OP, if you aren't happy with the 4890, and don't care about spending more money for a GTX285 and Physx, then why not? Seeing as you can get two 4890's for about the price of one GTX285 these days I would think that would be 'the' buy for a high end gaming system that doesn't break the bank, but if you really feeling you are missing out of Physx and just don't like the AMD drivers, then try out a GTX285. Good luck whatever you choose, though I do think if you don't mind spending some time tweaking it sounds like you can have most of the issues resolved. Either way I can't see truely being unhappy with a GTX285 or 4890 personally.
