Building a bad computer will not make your game lag. All you need is a nice internet connection. If you want it to look good however, I would research how to build a PC. I've seen builds on http://www.reddit.com/r/buildapc/ that are literally just about as much as an iPad that can play Skyrim or BF3 without too much trouble. Good luck!
That's what I thought too, but with $40 GPUs these days it's hard to get FPS lag on a game, and if you're going cheap, you're not going to be playing high settings anyway.
Honestly, the main thing you should focus on is to at least have an i5 processor. SWTOR, and games like WOW do not require a high end graphics card. So I would say, just as an example, to have an i5 and for your graphics card, pick up a EVGA 550. Might be slightly overkill but looking on newegg.com may help you out.
There are often "Build your own budget gaming PC" in magazines (often to suit varying budgets), computer shopper, even pcgamer sometimes. They are deffinately worth popping out and having a look to see if there are any in the current issues. They've done all the hard work for you, you just need to buy them and put it all together - which isn't as tough as some people think!