Why not brand new? If funds are short that is reason enough,but be cautious. I'm not convinced that your "old" one is SATA and almost certainly a DVD drive in in a 7 year old PC would be IDE.
You can make an improvement on old faithful as the other members have suggested, but if you are thinking of Win 7 it might be better to start from scratch and keep old faithful as a standby until it doesn't hurt too much to give it away or take it for recycling.
Before you finally make up your mind have a look at Novatech. enter link description here They offer every level of new, from real budget level through to the screamer you don't need.
Novatech too are honest traders with a long outstanding attitude of care about their customers and hope to see them back again for more.