I'm going to use an AMD Athlon 64 X2 4200+ - this is identical to one machine I already have. I picked this chip because it seems to be the fastest I can get that still runs with only 65 watts of power. It will go into an existing ATX case with a 300 watt power supply and only 2 drives - ATA100 @ 7200 and a DVD/CD writer.

I want as much integrated as possible for a couple reasons. One is power consumption. Another is that I don't want to fuss with the machine any more than I have to. Also, I don't care much about the graphics. Any graphics card that can handle boinc is going to be out of my price range.

The machine will mainly be used as a spare/secondary machine and for crunching. The Asus board I have it mounted in uses an nVidia chipset which seems fast enough and has no trouble handling 1024x768 resolution. I could get another Asus M2NPV-VM motherboard, but I'd rather try something new - if I'm reasonably sure that there aren't any known problems with the m/b.

I'm saving my pennies for when the AMD Barcelona quad cores come out - probably late summer or early fall (crossing my fingers, toes, legs, arms). But since I'll probably have to shoot my wad on just the chip, even then I'll be looking for an integrated m/b.

I finally finished getting prices for everything I would need and the total was almost $400 for the m/b, chip, 2gig ddr-800 (ouch), dvd writer and ata100 HDD. Whereas I can get another old Pentium up and running for almost nothing (a dvd writer). There's no comparison in terms of speed, but I think I'll be better off putting the money into building a Barcelona based machine. The $400 should cover the premium AMD will want when the chips start shipping.

I'm not normally on the bleeding edge of technology, but I'm really looking forward to this and don't want to skimp when the time comes. I'm talking 4 gig DDR-1200, 750gig hdd, maybe even a liquid cooling system.

I've been doing a lot of video conversions and even the X2 isn't cutting it. That's partly the problem of the software I'm using, but even so, by the time the Barcelona chip is out, there should be new software too.

I'm still going to need a motherboard though. I might try transplanting the new chip into the existing AM2 motherboard. If that seems to work, then I'll probably go with another Asus motherboard since that will mean that the new chip really is AM2 compatible and that there should be a minimum number of glitches. If that doesn' t work, I'll probably look seriously at Gigabyte and Biostar and get and AM2+ board - or whatever they decide to call them at that point. I don't like placing orders piecemeal like that but sometimes it just makes sense.

If only CompUSA weren't such vultures (no offense Team V). It seems they bend you over the check out counter every chance they get. If they weren't so greedy, I could do most of this with a 15 minute drive and not have to worry about returning stuff to Cali-for-neye-ay via UPSnail.

Pretty soon the 1TB drives will be out. I think Hitachi and Seagate plan to distribute these first. 750GB drives are good prices, but you might be better off getting two 320GB or 400GB drives and putting them in RAID 0. I have two SATA II 200GB drives striped in my work computer where speed is a must, and it works so much better than a single. If redundancy is your goal, then just put them in RAID 1. If you have SATA on your new mobo (which you will, everything does nowadays) then RAID support will be there, its native. As for the graphics I understand why you want to have on-board now. If it's not a gaming machine, what's the use to get multiple 512MB cards in SLI? You might want to spend a few extra bucks and upgrade your PSU though. 300 watts seems kinda low. But if you want your power consumption low, then it might be a keeper.
I agree with cargod on the Vista point. On-board graphics aren't good for Vista. Vista is very flashy and wants lots of video memory. If you haven't already got the X2 4200+ you might want to look at the 4800+. The 4800+ has dual 1MB cache and if I remember right the 4200+ only has dual 512KB. As for quad-core, I can always hope there will be one in my near future...
Tigerdirect is pretty good for parts, and I like newegg.com too. But don't buy computer cases online. Just go to your local store if you can. I bought a Sonata from newegg, the power supply was dead, and they wanted me to ship it back on my own dime, not just the psu, the whole thing! I said forget it and got an RMA from Antec directly for the PSU. I had a new one within a week and a half.
Good luck with the build, make sure and run a burn-in on it for a good while to stress test those shiny new parts. That way if they decide to take a dump early on in life, it will probably happen in the burn-in. Then they will be easier to get replaced under the warranty.

As long as you don't drill through power lines or plumbing, you're probably OK!
I have run ethernet cable all over my house, through walls, ceilings, floors, etc.
I have a 16" x 5/8" installers bit so it will go through just about anything... it doesn't make too big of a hole, but the ethernet cable will pass through fairly easily with or without the plug on the end.