I've been looking at it and it looks like BTC mining averages out to roughly 100MH/sec per $100-$150 spent.  It's not a bad investment until the ASICs hit the market and crush the GPU miners.

Litecoin mining seems to be about the same, 100KH/sec per $100-$150 spent.  It's not a bad investment until the ASICs hit the BTC market and all the GPU miners switch over to Litecoin mining and skyrocket the difficulty.

Both seem to have about a 3 month payoff period and its profit after that. 

So, basically your betting whether the ASICs will hit big time in the next three months or not.

If you think they will, you're better off just buying coins.

If you think they won't, go for mining.

If you're going to actually use the computer for gaming and mine during your non-gaming hours, then the mining is basically free and you should go for it.

Hope this helps!

 - Brian