Hi. I've looked up your PC specs here => http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=120&prodSeriesId=3677047&prodTypeId=12454&objectID=c01390590
and I can see you have a 250W Power Supply. Because this is an HP box, trying to upgrade *everything* is not going to be cost-effective, nor bring satisfaction. Either build a whole new PC, or else fit a medium-power PCI-e card (GTX550 max) http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/41590-nvidia-geforce-gtx-550-ti-1gb-review.html
and see how that performs for the online games you want to play. The worse that could happen is that you blow the feeble power supply and have to replace it with something better. HOWEVER - check power supply options and costs with HP first, because I'm not so sure that a generic PSU will fit in an HP case. Seriously, trying to upgrade HP (or DELL) PC's is a pain-in-the-ass because they rearrange the 'fit' of everything so that you have to go back to them to upgrade stuff. The 3 GB's of RAM in your HP box is enough, and I would not upgrade to WIN7 64bit unless building a new box with more than 4GB of RAM. The difference between 32-bit and 64bit on that HP PC would not be noticeable at all. One other thing - BEFORE paying for a video card to go in your HP, make sure it will physically fit in. I have a Dell that only takes a single-slot card. Good luck.