If your computer has a slot like THIS:
(just open your comp, take out card, look at slot, put back in)

LOOK CLOSELY AT THE SLOT
then a
EVGA GeForce 650 will work and is relatively small. (6" x 1" x "4)
Its $140 and it can run max S & S at 8000 bricks with no lag.
If your computer is 2 or more years old you will want to upgrade your power supply too. I recommend a
500w Thermaltake T2 power supply for just under $99
(its easy to install both, just unscrew card, pull little lever holding it in, pull card out, unscrew PSU (power supply), unplug all wires connecting components to PSU, the insert new PSU, plug in all wires, insert card, plug in power wire, Close case, plug into wall, turn on.)
If the screen looks horrible when you start your computer, that's normal. Just wait for the computer to recognize the GPU (windows 7) or go to EVGA.com and download the latest drivers there if it wont detect.
Feel free to get a better card, but beware, because the better the card, the longer it is
and it might not fit inside the case :P
Hope it helped.