Currently, unless you change your power supply, you are a bit limited on what you can choose. I run a Radeon HD 5450 in my Compaq Presario CQ5320F (low profile: you will probably need that decreased height as the power supply is a bit intrusive) Mine is a ASUS EAH5450 SILENT/DI/512MD2(LP), but you certainly can run DDR3, GDDR3, and GDDR5 in it without running out of power. Same thing with 1GB vs. 512MB. You can also probably use a Radeon HD 6450, but I cannot promise that it will work. You can also run a HD4350, but it is a bit lacking in power.
If you want to upgrade you power supply, I recommend a Corsair CX500 or Corsair CX600 (500W & 600W, respectively.) Not all power supplies fit into our case, but the guys at HP assure me that these fit in.
HOWEVER, you WILL need a low profile card, as the size of the Corsairs are a bit larger.
Hope this helps.