Re: $500-ish Ubuntu Box Questions
If you are going to drop $500 on a computer, don't get a celeron. A c2d could be had for very little more, and would be a huge step up in performance. If you were trying to build for like $150-$200 a celeron is a fine chip, but it will be a choke point on a system like that.
Other than that, the specs look good. I wouldn't plan on gaming on an 8400, but if you just want it for Compiz it will be fine.
Oh... be careful with these shuttle form factor PCs. The cases are EXTREMELY cramped, so it will make life very difficult for you when building (especially if this is your first time). A good atx mid-tower or even micro-atx minitower will give you space to work comfortably, and probably be both cheaper and give you more options for parts. Also, consider getting a new cpu cooling fan, as well as grabbing a tube of "artic silver thermal paste". There are other brands of thermal paste, and your new CPU will come with a stock heat sink. But these are not that good. If you want to protect your investment, I would strongly recommend this. A decent CPU cooler + artic silver is only like $25.
Desktop: AMD Athlon64 X2 3600+, Nvidia 8600GT, 3GB RAM, 80GB hd, Windows 7 Beta
Lappy: Sony Vaio FW-140E, Intel P8400 2.26Ghz, 3GB Ram, 250GB HD, Intel x4500MHD, Windows 7 Beta & Kubuntu 8.10 w/ KDE 4.2