GeForce GT 640 optiplex 755 mt

if you know how to open up your case, you will be able to find that out yourself. here is how. in order for a gt 640 to work in your PC, your motherboard will need to have a PCI express 16 lane. you can just google the term and you will find out what it looks like. then you open up your chassis. if you are able to find the lane on your motherboard, then you will able to install it.

BTW the good thing about GT 640 is that it get all its power from the lane, so there is no extra power connector needed.

Good luck!
 

Also don't get a 640. It's a very BAD bang for your buck card. Try out a 7950 for $130 on eBay.
Good Luck!
 
I really don't think that you can get a fully functional 7950 for that price. but if you can, that will be an exceptionally good deal. but for rthunderbird, you will have to get a better power supply if you want a 7950. 7950s require one 6 pin and one 8 pin connectors from your power supply.

 

You can search ebay and you will find many. only 2 6pin needed. 430W PSU is good.