The X4 system would probably be your best bet.
This is what I was thinking, but my problem is that it's not the old Athlon II X4, it seems as though it's the new A8-3850 but with the IGP disabled. These Athlon ii's have only be about for a couple of months so the benchmarks available on http://www.cpubenchmark.net
are not particularly accurate due to small sample size.
I currently have an old/original Athlon II X3 in my desktop machine running Ubuntu. As well as the usual desktop tasks (multiple browser instances, email clients editors, uml tools), I also run a local MySQL server and Apache server. I've tested the Magento store on my desktop and it's pretty responsive, more than good enough for development. Therefore If I could get an original Athlon II X3 or X4 then I wouldn't hesitate to buy and start the build now, but there are non available and I'm not convinced that these new Athlon ii's will be as good as the original ones.
I'm wondering how much performance is lost from the A8 with the IGP and whether this will have much of an impact on a server.
Do not spend ?20 on a case and a PSU, the case will rip you to pieces as you install the components in it and the PSU will last 1 day more than the warranty period and then explode spectacularly and take everything else it's attached to with it! OK, maybe a bit of an exaggeration but possible - failing PSUs have been known to blow out motherboard and disks as they go.
I've lived without brushed aluminum cases for the last 15 years, so I guess I can put up with some rough edges for a little while longer. The first server I built lived in a cardboard box for three years and never complained.
Never had a PSU blow up or hear first hand of a PSU blow up, I've had them die at my parents house by sucking in too much cat hair, but every one which has died has always done silently without taking anything else with them. Only motherboard I've ever killed was through my own stupidity/lazyness, dropped a screw in the case, didn't bother to fish it out and eventually it lodged itself between the motherboard and case and shorted something out with lots of smoke, but the cpu, ram and disk were still fine. The reason I'm saying this is that I think that sometimes the more expensive stuff is just the same as the cheaper stuff, just with more stickers on it and a spray of gold paint. This project is not for a production server, it's purely web development and possibly a mail server.
Basically I'm more than happy with my estimates for all other components (I'll live with cheap stuff for now) and have a budget of approx ?70/$110 for the processor and was after advice on which processor will offer the best performance for a web/mail server within this budget.