Hi!

 

My suggestion is:

On the cpu, go for an INTEL CORE I7 cpu so that you can benefit from Posers multithreading when rendering (dont buy the latest version of the cpu)

Of course you need a matching Motherboard that fits the cpu, "Asus" makes good and rather non expensive ones.

You may have to buy new sticks of RAM that suits the motherboard too  (you say you have 4Gb but i dot now wich kind) but its not so expensive.

On my Asus motherboard i started with 3 x 2Gb sticks for RAM and that was quite enough (my board demanded the use of 3 sticks of ram)

Hard drive, buy (NEW) 2 x 1Tb at 7200rpm so you have 2 internal drives, one for daily use and  one for backup.

This is my suggestion for a bugdet comp.