Re: what to buy?!?
If your motherboard doesn't support DDR 2 and SATA upgrade is not a good option. DDR (1) memory is more expensive, slower and has lower capacity, the same is true for PATA hard drives. Your processor is probably Socket 478, I don't think they make new ones like that, you can only hope to find second hand one. Maybe you can find one with 2.8 GHz, but that won't help much.
My recommendation is to by a new PC. If you want to keep this one you can use it as a server, or connect it to a TV and use it for multimedia.