The very best 5.1 systems do approach the performance of good stereo, but you are talking big bucks. A Lexicon processor coupled with Bryston power amps and PMC loudspeakers is a prime example of this. But that won't give you much change from £10,000 (and this is the baseline).
A better option, perhaps, is to start with the two channel and build up to 5.1. Many stereo amps include a processor add-on option (download Arcam's A70 manual to see how its done in some cases) and companies like Cyrus design stereo and multichannel to sit comfortably together.
A final option is to do without surround altogether. If you are really keen on stereo, go with stereo. Surround will drain budget that could be best spent on better stereo. You lose out on rear and centre channels and subwoofing, but well-sorted stereo will do surprisingly well at overcoming these 'limitations'.
Others will disagree, of course...
