I can understand the preference for lots of working space, but there's a certain sense of pride in
getting a fully working PC in to a nice small form case
If you'd be happy with a 4350 (basically the same as the 4550 but with DDR2 memory rather than DDR3) you could save yourself a few quid - they are findable for nearer £25. If you're happy spending £30 then a 5450 would be ideal.
If you can wait, however, AMD have just released the new 6-series low end cards. They're currently OEM only, but I'd be surprised if they didn't filter through to retail in the next couple of months, and the bottom end of that series has double the power of the 4550/5450...