CPU mining is pretty much pointless now. And unless you're not paying for power GPU mining is pretty much a wash. Assuming an ATI 5870 uses 200W, and can do 400MH/s (which looking online seems about correct), you can expect to generate about .01 BTC/day, which is about $1 at current rates. You'd be looking at profitability if your marginal power costs are less than $0.20/kWh. And that's not factoring in the cost of the graphics card.

For comparison, 2 USB asic miner erupters will hash at about 650MH/s, and use about 3W.

So in short, if you already have a fast graphics card and don't have to pay for power, it might be worth mining.