Definitely another 1GB card, so that you keep your 1GB of vram instead of doubling your core power and halving your memory amount. That would be silly. Any GTS250 1GB card will work fine, brand doesn't matter.
Definitely go for a better single card instead of getting another GTS250. Like Zealot says, the GTX460 is a good choice. There's also the Twin Frozr II 465 at newegg for $175 after rebate right now. That would be a great choice as well
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