An alternative view,
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I've built two this winter both using a large number of existing "bits" such as monitors, keyboards, mice, HDDs and sound cards. Floppy drives went deep storage; one USB FDD is all that's needed now. Don't shy away from the task; it's really getting easier as time goes by.
Machine A is a low-range ShuttleX with bits from Overclockers and cost £350-400. (Spare me getting out the invoices to be precise!). I like the small footprint and low noise once I got a grip on the fan speeds. Powerful BIOS gives lots of flexibility. Excellent instructions may mean you'll be wasting £50 if you let your IBM chum make it and you'll deprive youself of the fun.
Machine B is a high-range Abit MoBo in a fancy case with a new Hercules graphics card that alone cost as much as the ShuttleX complete. (I do a lot of Graphics/Digital video stuff). Extra cost incurred also for the DVD-RW and the two SATA 120GB HDDs that may get "RAIDed" when I'm ready. Again, no floppy but the old DVD-R/CD-RW drive gives capability for DVD/CD copying when required and possible. Same comment about instructions; excellent. Everything provided down to the last screw.
You can't save by using old RAM. Get a good MoBo and Processor and accept that you are going to need at least 512MB but 1GB would be better.
"Future-proof" it with leading edge spec components and it'll last you 3-5 years.
But then there's the new Operating System and Applications to run on it.... They're going to have to be up to the standard of the Hardware.
At least you get some of those in the shop model.
Overall? Go for it. fobs