I'd use CPU-Z (Google it), but we do need to know the CPU and memory.
Another point is the powersupply, most prebuilts have weak PSUs which will limit upgrade possibilities. Open the case and look at the PSU label, we need to know: Wattage and output on the +12v line or lines.
Also is the case normal sized or 'slimline' if it's a slim case you'll need a low profile card, which can further limit the upgrade options.
i'm kind of new to all this so bear with me i really dont know what a gpu is but i have GMA 3100 and an intel eleron cpu 450 @ 2.20GHz 2.19GHz, but i plan on upgrading to a Core 2 Duo E7xxx (Wolfdale-M) (65W). the psu says output +12V/14.0A,-12V/0.8Aand +5v &+12V OUTPUT POWER CANT EXCEED 218 w. with a intel G31 Express chipset and 2 ghz of ddr2 ram and just a regular case. i am trying to spend 150-200 dollars.Is this enough info or do you need more? Thanks.
First the bad news: That PSU is just not enough, it will have to be upgraded to cope with even a limited graphics upgrade.
Now the good news: Your case seems to be a standard width, so most cards will fit, but you need to measure the distance between the back of the case and the front to see what length of card will actually fit. Also prices are beginning to fall, so you might bag a bargain if you shop wisely
Here is how to check the card length:
Get the photo from Newegg and paste it into a graphics progam like Paint. Cut out the PCI-E edge connector-this is 85 m.m long overall-and paste copies along the length of the card image. With a little math you can calculate the exact length of the card by scaling this way. Also look carefully at the case and motherboard, most powerful cards take up two slots of space, a problem for some.
While there is HD4870s for this price, they all have crappy coolers that dump their heat into the case or have loud fans, or both: