PC2100 DRR SDRAM (DDR266).
Dude, this is a very low end machine. Its a 2.4ghz Celeron with a 400mhz FSB. Your machine also does NOT have an AGP alot according to the specs I found. I suggest you suck it up for now and do a full upgrade on a PC when you save enough. Its not worth upgrading at all to play games. If anything, buy 256MB DDR266 Memory to help speed with internet browsing and other basic activities.
A+, Network + , HP Certified Tech and MCP
Specs: AMD Phenom II X6 1095T, Asus M477TD, 8GB GSkill Ripjaws DDR3 1600 7-8-7-24 1T, 128GB Crucial M4 SSD, ATi HD4650, W7, 27" HL272 Monitor