What graphics card do i need?

I have a fairly old system its a packard bell ixtreme and im looking to put in a new graphics card fro gaming. currently it hasen't got one (well if it does i cant see it) but i thnk i would need a AGP, but i bought a AGP one before but it wouldnt fit so i took it back.
Heres all the details of my motherboard and stuff, sorry if ive missed anytn out:

Operating System
Windows XP Home Edition Service Pack 3 (build 2600)

System Model
Packard Bell NEC Packard Bell Computer P851008201
System Serial Number: 030867420280
Chassis Serial Number: 50424E
Enclosure Type: Desktop

Processor a
2.60 gigahertz Intel Celeron
8 kilobyte primary memory cache
128 kilobyte secondary memory cache

Main Circuit Board b
Board: NEC COMPUTERS INTERNATIONAL GA-8SIMLNF 1.0
Bus Clock: 100 megahertz
BIOS: American Megatrends Inc. 1.0M 08/15/2003


Display
SiS 650_651_M650_M652_740 [Display adapter]
Packard Bell NEC CI FC700 [Monitor] (15.7"vis, s/n 310336523020, August 2003)