Well, if you can try upgrading and get an old AMD. It all depends on your motherboard and what processors it can take. If you upgrade to the new duo core processors, you will need a new motherboard, which will mean a new power supply, which means new memory/RAM.
I suggest having a good look to see what you can get for your budget. You dont have to buy a brand new system but one that offers the basics is a good start. Whatever you do, avoid getting Vista on anything below 2GB of RAM