! I have to admit, your computer is very old and it would not be very cost efficient to upgrade it as it stands.
I'm pretty sure your computer would be a socket 478, although it doesn't say on the Compaq website. If so, the most you could upgrade to would be a 2.20GHz processor, not a huge increase, and as it's an old technology, you possibly won't be able to buy one new, which means scouring second-hand stores for a non-faulty one...
Your graphics card is seated in an AGP (4x) slot, so as long as the upgraded graphics card meets this specification, it should be alright. AGP is also an outdated technology.
I don't know your budget limitations, but I personally feel it would be much better upgrading pretty much the whole rig - new motherboard, processor, RAM, video card, and power supply. You could possibly keep your current hard drive, optical (CD or DVD) drive, floppy drive, keyboard and mouse, monitor, speakers, and case. It would be possible to do this fairly cheaply, but again, considering your budget, I'll await your response.