I own a Dimension E521 and the only games i can play are Half life (The original) Counter-strike Source (but it runs poorly) and Team fortress classic. what kind of specs do i need to at least run Team Fortress 2?
Minimum? I would RECOMEND, for a good entry level rig, an AMD anthalon X2 5000, an ATi Radeon 4850 for the card, any decent Socket AM2 motherboard, such as the ASUS M2N-e SLi and a 500 watt power supply.
If you just want to purchase a computer, just make sure it has a dedicated GPU that is fairly good and your should be able to run those games just fine.
You could put a HD4670 or HD5570 in that computer without needing to worry about power. Either would be a massive improvement over integrated graphics. If you have only 1GB of RAM, getting another 1GB would probably help too.
Edit: But, as Jack says, we need a budget (and your location, or where you can buy parts) to be able to give the most thorough advice.
Well then, I suggest you start a new thread in the New Build section of the forum, using the format in the sticky there. A budget of $800-$850 could build a better gamer than that Dell could ever hope to be.