I'm a box builder, and don't have much time for off-brand stuff and the early failures and messing around that go with it. Stick with well-known, reputable vendors and you will be much happier.
First, depending on the size of your LAN, a dual-processor machine running linux is like night and day compared to a uniprocessor box. For a dual proc, I like the Tyan S2466, which you can get on the net for little more than $200. For single processors, any good ASUS board will do fine with SME, but I do prefer the AMD cpu's, as dollar for dollar, they are faster, better, sturdier, etc than pentiums.
Use good memory, get it at Crucial.com, or equivalent. 1GB isn't out of the question, and your network will thank you.
If you can afford and justify raid, get a 3ware card and drives from Western Digital with 8MB of cache. I always do a raid array plus a separate boot drive. 3Ware has a new 2-channel SATA controller and WD has great drives to go with it.
Hope this helps,
Tom