Graphic card compatibility

Hey guys, I have a question. About 6 months ago I purchased a computer from Alienware, the Area 51 750i. Anyways, I'm just starting to get into computers and am stumped when it comes to checking compatibility of a graphics card because I keep seeing 16x and 2.0. I have a 750i SLI motherboard currently using a nvidia GeForce 9800 GT, and want to upgrade it. I have absolutely no idea on how to check the compatibilty for what I want to get (I'm looking for DDR3 and 1gb, and might use 2 graphics cards if its within my budget. Any help would be greatly appreciated. Thanks!

The exact motherboard and PSU specs are also importaint; Dual cards can draw a lot of power and not all motherboards support SLI at full speed.
BTW, two 9800GT is still a good combination!

most all new graphics cards are PCI express 2.0 x16... x16 is the size of the slot and 2.0 is the technology, I dont think you have to worry about those for compatibility, if your motherboard has an extra slot that looks the same has the one your 9800gt is in, then don't worry cause 1.0 PCI is pretty much dead (like usb 1.1)also you must check the connections on your psu and the wattage, the spec page for any card should list the minimum wattage and what connectors it needs (some need 2)