For Bitcoin, forget using a GPU or Block Erupters because the difficulty is now so high that you'll get almost nothing from them.

As an example, I have 2 block erupters that I've been running for 3 months and I've made around 0.25. Now with current difficulty, I make 0.01 BTC every 3 weeks and in 4 weeks time it'll probably be half that so even if I left them running another year then I'd probably only make another 0.03 in total.

Luckily with BTC at $1,100 I've made a little profit on the one that I paid £60 for but a big loss on the one that I paid 0.9 BTC for (about the same amount at the time).

You'll need to buy something with 100's GH/s to accumulate any significant BTC, and that'll cost you $1000's. If you think BTC will continue it's exponential growth then just buy BTC directly. The only certainty is that difficulty will continue to grow exponentially so mining is pretty much a loser all round.