for the price of what you would be buying off ebay, you could get a similar unit through dell with a warranty, if you didnt want to build one yourself.
If you want to build one, you need to have in mind what you want to do with it. -general web surfing, a bit of games, or heavy games, or graphics work, etc.... more games or graphics work will require a higher end video card, and more ram.... if it comes down to a question on spending a few extra bucks on a better processor, or on more ram, GET MORE RAM, and get a decent processor with the most L2 cache you can get in your price range.
as for brands? yeah ECS boards arent really any good-- kinda like the chrysler's of the motherboard world... they look pretty, some people dont have a problem, but all in all, they're crappy. --if you're going intel system intel makes good stable boards. amd wise, you've got a few choices... DFI, Biostar, Gigabyte, and asus.... DFI has some awesome products--i love mine. I have also had good luck with biostar's. the asus boards i have used, i have not been impressed with, especially considering the price hike for the name.
there's still a good bit of disagreement on which brand of processor to buy, but if youre building you own box, amd's are still cheaper for the most part. you wont really see a cost savings in intel vs amd in prebuilt systems, usually.
Memory--
the PC5400 memory (667mhz) should be your lowest speed option, but you will need to make sure that the motherboard you pick out will support your memory choice. --- if you plan to only websurf and type documents, minimum 1gig ram.... if you plan to do other stuff, minimum 2 gig. --having enough memory is key --- stick with kingston, crucial, ocz, or even ultra if need be, as far as brands go.
the video card is another thing that will vary depending on what you do with the system, but a good nvidia board with minimum 128 meg memory, reccomended 256.
the dvd/cd drives are a bit more trivial at this point in the technology market, but typically, you'l end up looking at a nec or toshiba drive... dont go sony though.. too many problems with those-- just make sure it matches the case.
cases and power supplies-
power supply... 350watt minimum. if you plan multiple hard drives, optical drives, you'll want to be in the 450 and up wattage class, but if you dont plan more than 3 or so drives total, dont waste your money on wattage.
as for the case... it's a bit more subjective, but you get what you pay for. look inside the case for good construction (rounded edges/corners, lack of sharp edges) and tool less design is always a plus. I dont like bay doors on the front.. they get broken SO EASILY you dont neccessarily have to go overboard on the case, like an all aluminum one or what not, but shop around and compare. -- and make sure it has enough drive bays for your components
as for the HD, I'd say minimum 160 gig. if you do music/movies, go 250 or more. -- seagate is getting better, maxtor is too, western digital is kinda eh... i've had the most failure with WD's as of late. Hitatchi is actually quite good (they bought ibm's hd line a few years back)
as for the OS, XP is fine. XP MCE is OK too, if you need the MCE features, but if you dont, dont worry about MCE.
Vista is still young. If you get a pre built system, and they offer it to you, i'd try it, but only if the machine you are getting will handle it well...
so yeah, i think this is my longest post ever on here.. but im a pc tech by trade, and im bored... so yeah hope you had fun reading!