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It's been mentioned elsewhere on this site that a USB microphone will cut out signal noise but I've noticed there's an irritating latency with mine, that I discovered when trying to turn my computer into a karaoke machine. It depends, then, on what you're looking for; if it's for video chat or teamspeak, I'd go with a cheapo analogue one. If you're looking to do voice-over our audio blogging on a budget, go with USB.
I can't imagine there's anyone who uses .gs who's sampled a wide range of budget microphones. I think you might have to pop down Maplins and just buy whatever cheapest mic they have and hope it's good.