That's a shame!

I'm not sure about used prices in Brazil, but take a look at what you can get with your budget. You will definitely want to upgrade both the GPU and CPU, if you want to play games, but you could try to go for "balance"- something like a Core 2 Duo paired with a HD4670. It all depends on what your local prices are like. eBay is definitely the way to go, I'm afraid- you won't get the parts you need new. But eBay does have lots of protection for the buyer- if the product doesn't match its description, you can always get a refund through Paypal. Just be careful what you buy, make sure it will be compatible before you buy it, and make sure it's described as being in good working order with no defects.
A word on upgrading processor- even though the Core 2 processors have lower clock speeds than the Pentium 4, they go
much faster and are much better choices for gaming on. Take a read of this page if you want to find out more:
http://www.anandtech.com/show/2045 Even the Pentium D, which is basically a pair of Pentium 4 working together, gets thoroughly beaten by even the first generation Core 2 Duo.