I dont see any specs on your current power supply either, but on the rear back of the case on the psu or inside the case on the side of the pcsu you should find a stiker telling you the information you need.
If you do truly want a new video card/power supply there a tons of places to get them. Installing them is realtivey easy, just remember to turn off the pc first.
For the psu it's pretty straight forward, there are 4 to 6 screws on the rear of the pc, around the edge of the psu, remove these and hang on to them.
Make note of where your psu connections are. For example 1 to each hard disk, one to the 3.5 floppy, cd rom, motherboard and fans that require a psu connection.I personally use a notepad, and on occasion my camera to help remeber these things. now gently remove all the power connections from the devices and mobo. remove the old psu and install the new one. to install the video card (since you have none to remove) is pretty simple as well. find it's pcie slot, remove the tin cover and gently but firmly put in place, it should drop straight into the slot and require like 10-15 pounds of steady pressure or less to pop it in. run the appropriate power supply cord, and your done.
If you wish to shop for psu's and video card for price vs performance etc. try cnet.com or ask here.
I currently have a Zalman zm850 hp with a pair of 9800 gtx+oc running on a duo core 3.0ghx and 4mb ram, it plays wow and everquest seamlessly at 100fps and 0 video lag on high settings. with the 9800 tho i recommend getting evga precision tools as their drivers for it let the fans run only at 40% with the tools i can turn them to 100% while playing, and do minor tweaks for performance.