neblix, you don't know anything about computers. don't sound like you do. if phill already didn't know as much as he does about them (he's got me way beat in the software department), he might almost think you knew something and get worried or something. like, mounting points...do you even know when those standards came around? like 13 years ago, back when AT was being overworked and they needed a new design for a mobo. AT and ATX share four mounting points, regardless - that's enough for a computer that isn't being shuffled around a lot. phill and i go way back, i'll handle this
cases are cases. who gives a shit about them? just make sure your fans spin. my old computer was in an AT desktop case with a 486 sticker on the front. same thing with optical drives - as long as it works, don't complain. hard drives are fine as long as you keep track of how old they are and test them a lot.
in general, i'd say to go with the i7-860 before you go with an i5-anything. way better processor for only a few bucks more.
i'm not a fan of mushkin that much. if you want performance ram, just get g.skill ripjaws, they're a good balance of price and performance.
the GTS 250 is a very capable card. if you've got the money, spring for a core 216, as there's a huge increase in performance. also, how big's your screen (resolution)? if you're running something 1680x1050 or above, spend the extra five or ten bucks and get a 1gb card. big screen resolutions benefit from more vram more than they benefit from overclocking or anything else.
no mention of a power supply in there. use this to figure out how much wattage minimum you'll need, and this to find what you're looking for there. remember, the number the psu calculator puts out is how much wattage you need, not what the psu should be rated. if the psuc says 300w, you'd have to get a 400wpsu with 75% efficiency to meet the minimum standard (which probably wouldn't hold up under load or on boot too well). i have a core 216, an i7-860, 4gb of ram, one optical drive, a high-end motherboard, and four hard drives, and i have a seasonic x750 750w psu (which is a little big for what i've got). you could probably go 650 or so if you don't have a ton of hard drives or go with the 250.