Well, I know you already bought something, but I have the analog (soundcard) version of the Medusa 5.1 surround headphones:
http://www.medusa-usa.com/product_info.php?products_id=777I've been using them for a year, and can attest that they are quite incredible. The surround effect is very noticable, but not entirely useful, in Aces High. For first-person shooters like Counter Strike, it is totally unbelievable and quite a tactical advantage.
When you combine it with a TrackIR, then AH does take on a whole new level: as you turn your head to look around, your engine noise moves around too. When you look backwards, your engine noises seem to be coming from behind.
The effect is slightly useful for locating GVs when their engines are running, but for normal flight, it isn't terribly useful. Cool, yes. Useful, not totally.
-Llama