I usually run a tight budget and so I generally go for a little lesser powered system and save $$$'s.
My suggestion would be to go for a system built around the AMD 2400+ because it's a processor with adequate power for most applications, and for the price it beats most everything on the market right now. I checked pricewatch and you can get one OEM for ~74 with shipping.
Then you'd need a mobo that can handle the processor. What I usually do it fish around ebay for some former supergamer mobo that someone is trying to get rid of. You can even get one with the RAM and processor included sometimes, which makes it an even better deal (as long as it's guaranteed not DOA).
Then I always reuse my case, a cheap old ATX, but hey it works.
Then throw in a decent video card (geforce 4200? only $90) that's a great price vs. performance, use your old drives (or even get a little older, slower speed dvd burner for $120, and that way you can burn cd's and dvd's, rip anything and watch movies).
As far as noise goes, I can't stand loud ass computers. It just drives me crazy. Not too long ago, my computer, too, suffered from "gross acoustical overload," so I went hunting around for something to fix it. What I came across is a great little fan called the vantec stealth line. That in combination with a cheap solid copper heatsink is Sooooo quiet. And it you don't use really hot drives and/or have a good quiet video card, then you don't really need case fans either. Some people might disagree with this, because of the higher heat that builds in the heatsink because of the low speed of this fan, but if you don't overclock, who cares?
I also can't really stand WinXP or regular 2K or 98SE, so I found an OS addon of sorts for 98SE that really improved my overall system performance dramatically called 98lite. I HIGHLY recommend it. It guts the running html engine out of the desktop interface, and slices out alot of other unnecessary crap. Really a cool program. And it's free. On my 1.2 athlon system I wanted to play Comanche4, but in XP it wouldn't even run, and in 98SE it was a total slide show. With 98lite I'm getting about 25fps on average. How bout them apples. Plus file browsing has zero wait time no matter how fast. Anyway, it's a great program.
You might also consider a cheap RAID array, too, if you've got a few bucks left over.
Motherboard - MSI K7N2GL 99.00
Processor - Athlon XP 1700+ --- 42.00
Ram - Corsair CMX256A-3200C2 --- 60.00
Hard Disk - WD ATA/100 40GB 2MB 7200RPM --- 57.00
CD-RW - Lite-On 52x24x52x CD-RW --- 46.00
Floppy - Teac 1.44 Floppy Drive --- 12.00
Speakers - Juster 5d-107 7 PCS --- 34.00
Case - Raidmax Model 208 10 Bay 350W PS --- 43.00
Total -=- 379.00 inc s&h.
www.pricewatch.com or www.newegg.com (prices are similar within a dollar or two in most cases. Also need to add a fan to this mix but I left it off... people are funny bout their fans especially if they get into overclocking.)
This is a good overclocker, it's pretty decent quality, its upgradeable, etc...
I agree with just upgrading the cpu/mb/ram on that budget.
I pretty good AMD cpu/mb/ram can be had for $200-$300. I would use the extra cash to buy a good quiet heatsink/fan for it, possibly new QUALITY and quiet powersupply, and possibly a quiet case fan if necessary.
BTW, the 300-400 budget is for the mobo/processor/fan by themselves. I already have a gig of DDR ram, and a sweet new aluminum case. Graphics card-wise, I am going to get a Radeon 9700 Pro, so I'm set in that regard as well.
I always had a problem with heat with Athlons, so the case is equiped with silent fans out the wazoo. I still need a good processor cooler, though. The one I have is the Dragon Orb, two stacked heatsinks and fans; it works pretty well, but is soo sooo loud. Anyone have any good suggestions? It shouldn't be too extravagant, as I figure by next christmas I will have enough funds to switch to water-cooled... maybe that will even let me overclock it.
As far as power supply goes, a 350 came with the case, so I think that will work well enough.