PHYS-X graphics card

to all you pc gamers out there, i was thinking of adding a separate graphics card to my pc for dedicated phys-x support i was thinking of getting [www.tigerdirect.com] or

[www.tigerdirect.com] not sure if i should go with a 512 mb or 1gb card? my pc manufacturer said to stay within the 350 watt minimum psu wattage requirement. if there are any other cards between $50 and $100 that might be better for phys-x please let me know. my current pc specs are below. i know i have an excellent graphics card i am only asking this because when i play batman arkham asylum or mafia 2 demo and turn the phys-x to max it drops the framerate significantly (especially while playing in 3d) so i figure if older video cards are cheap why not get one. thanks for any help,

Motherboard: EVGA P55 FTW
Processor: Intel Core i5 760 2.8GHz LGA 1156 Quad-Core Processor (8MB L3 Cache)
Power Supply: 750 Watt Corsair PSU
Graphics Card: Single 1536MB GDDR5 Nvidia GTX 480
Memory: 8GB Corsair XMS3 DDR3 1333Mhz (4x 2GB) Dual Channel Memory
Hard Drive One: 80GB Intel X25-M - Solid State Drive
Hard Drive Two: 1.5TB- SATA-II, 3Gb/s, 7,200RPM, 32MB Cache HDD
Nividia 3d vision

for a physix gpu the following is probably one of the best ones:

http://www.newegg.com/Product/Product.aspx?Item=N82E16814150479&cm_re=gts_250-_-14-150-479-_-Product

gts 250/9800gtx+ are both amazing physix gpu's paired with a good partner you can not go wrong.. hence why evga had the 295 and 275 co-op editions.

thats all i need to say, BAM!