I don't think there's a better 5.1 set than the old Medusas in that price range.
I quite like the old Medusas - I have a set myself but I'm concerned that they're getting expensive. When I bought my 3 or 4 years ago they came with an amp and cost about £40. Now the version with an amp will set you back £60.
A word of warning though. I'd been baffled by the number of people complaining about them recently. I've just found this review of the ProGamer edition by someone that used the originals:
http://hwzone.co.il/english-reviews/progamer-eng/5/
In short - they're not as good as they used to be. It sounds like Speedlink have skimped on the quality of the drivers for the centre and rear channels. (I expect a few people will have a chuckle at this info coming from me

).
Shame really. A lot of people like Dolby Headphone on Xonar or CMSS-3D headphone on X-Fi via stereo sets. Presonally I don't find them convincing enough.
When a soundcard is available that uses an algorithm as convincing as this Youtube clip (using Cetera algrithm from Starky hearing aids), I'll be happy to get rid of my medusas.
http://www.youtube.com/watch?v=SCwyJ8bKp9o&fmt=18 (listen on stereo headphones)
Edit: The current medusas are overpriced. Avoid. Sorry I don't have another 5.1 set to recommend.
Further edit: Reading some reviews, I gather that the home edition with the amp may be better. But still - £60?
Even Further Edit: OH THE CHUFFING IRONY. I've just bust my Medusas. The pin for the sub channel has come off the proprietary connector to the headphone amp. I have no bass. Time to test out my n00b soldering skillz.