How much does it cost to upgrade a computer?

[Ben]

Ok so recently I have decided to give my computer an upgrade. It is a very old gaming computer and totally out dated. I am short on money so I would like to get a good upgrade but still in a small budget which I have not decided yet. I don't remember all the specs of the computer. I had the graphics card upgraded recently so I know that does not need an upgrade but the rest does.

My specs I remember or know.
AMD Athlon 64 X2 dual core processor
2gb Ram
Gigabyte S-series GA-MA790X-DS4 motherboard
Gigabyte GeForce GTX 650 (I am pretty sure)
Windows XP Home edition service pack 3

I have know idea if I did all that right as I am not computer savvy and it has taken a while to find all this stuff. I play the games not the computers if that makes any sense at all haha.
My case is 19 inches long and 8 inches wide if that is the measurements you are meant to useand I have no idea what the power supply is. Like I said I am not computer savvy. I don't mind not running games like FPS on ultra settings as I am not really an FPS kinda guy but I really love RTS and RPGs of all sorts and various other adventure games I guess you could call them. I get that these games are much bigger so the graphics are generally not as good and the card I have runs them pretty high but still slows a bit because of my other specs so anything that can run those kind of games I would be happy. I am definitely getting Windows 7 because so many new games have come out requiring vista or above and many of these games I miss out on because I can't play them.