Graphics card upgrade from Geforce GTS 250

APPROXIMATE PURCHASE DATE: within 1 to 2 months BUDGET RANGE: card must me <= $500.00 before rebates

USAGE FROM MOST TO LEAST IMPORTANT: The card will be for gaming at extreme settings, i want to be able to run future games on higher settings at least for a while before upgrading.

CURRENT GPU AND POWER SUPPLY: nvidia geforce gts 250 1gb ddr3 RAM, 750 mhz shader clock, PSU is 850W

OTHER RELEVANT SYSTEM SPECS: CPU is core i7 950 3.06 ghz, 6gb RAM, mobo is gigabyte x58-usb3, case runs cool

PREFERRED WEBSITE(S) FOR PARTS: newegg, tiger direct etc

PARTS PREFERENCES: I dont care, I have always used Nvidia cards, but im starting to wonder about the "fermi" architecture or whatever, ive read things about nvidia overheating lately. That being said, ive read about ATI's card having bad drivers and causing really low frames. In all i dont wan to go cheaper quality, but if you can give me good reason to use a particular brand that would be nice

OVERCLOCKING: Is it ok to overclock a card right out of box, doesnt it void warranty?, I never really mess with overclocking, but I can if i know more about it. I will SLI/Crossfire eventually, but ive always wondered if its worth it, or rather just buy one card, and then keep upgrading with just one card

MONITOR RESOLUTION: something that can support at least a 24 inch Full HD.

ADDITIONAL COMMENTS: Sorry if a lot of these questions sound noobish, And thanks for any replies and advice given.