I bought a couple of ASICMiner block erupters off eBay for about $40 each. They do 335kH each.
Decent enough to get you started in pool mining on any SHA-256 currency, I think BitCoin has just too high a barrier to get into right now but that's just my 2 Lincolns

Plus if you're new to all this (like me

) It's best to keep your investment low until you know what you're doing with a command line and compiling code.
I think even to pool mine BTC you would need a minimum of 5gH just to even make it psychologically viable. But I'm a newb at this and it's just my Google-ated guess.

Its better to not waste potential investment money just buying hardware because you want to. Wait for a good opportunity and jump on it, usb block erupters are not one of those. Also you dont need any minimum amount for pooled mining, that's the whole point of pooled mining...