I was wondering if you could help me… I have this desktop http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1607529
The current graphic card is an integrated one (with the processor) its Intel HD 2000 which is not that great for gaming.
I want to buy a graphic card for it but I don't know what kind of slot I have to use: is it PCI / Pci Express?
That Thinkcentre has a 2nd-generation Core i5 2500 processor, so it has version 2.0 PCI-E x16 slot, which is compatible with any PCI-E x16 graphics card on the market. However the power supply isn't very strong.
According to the specs on Tiger Direct that computer has a 280 watt power supply. While that's better than 220 watts, it's still only enough to support low-end graphics cards like a Radeon HD 6450 or GeForce GT 610. Those are $50 models which are just slightly better than your integrated graphics.
You'd want to upgraade the power supply in order to install cards with decent gaming performance.
For example, this PSU:
http://www.newegg.com/...6817182074
Can support entry-level gaming cards like the Radeon HD 7750.
http://www.newegg.com/...6814102969
While this PSU:
http://www.newegg.com/...6817139027
Can support cards up to the $220 level, like the GeForce GTX 650 Ti/GTX 660 and Radeon HD 7850/HD 7870.
There doesn't seem to be much info out there on that pc but I believe it has an PCIe x 16 slot but which version I can't seem to find.
If I may suggest that you check the pc's manual to see what it says.
I also found that it only has a 220watt power supply unit with limited connectivity so you will most likely need to upgrade that for a video card.