If you replace the motherboard you will have to reinstall everything, period. There's no way around that fact, unless you replace with an identical motherboard/processor.
I looked at newegg and zinglebarb's right, there are new motherboards with AGP slots.
Though I wouldn't call it new though, I found you this one, fairly cheap and socket A so you can reuse your old processor (and maybe save yourself from reinstalling Windows, I'm not too sure...)

Newegg.com - PC CHIPS M848A (V5.0) 462(A) SiS 746FX ATX AMD Motherboard - AMD Motherboards
Or for more upgradability options in the future, there's this one, which features AGP & PCI-E as well as DDR & DDR2 slots
Newegg.com - ASRock 4CoreDual-SATA2 LGA 775 VIA PT880 Pro/PT880 Ultra ATX Intel Motherboard - Intel Motherboards

If you are looking for a cheap system, i recommend this. And for those who complain about certain components, remember, i always do my research. (coughpowersupplycough)

Newegg.com - Thermaltake Swing VB6000BWS Black 0.8 mm SECC Steel ATX Mid Tower Computer Case - Computer Cases

Newegg.com - GIGABYTE GA-MA69G-S3H AM2 AMD 690G HDMI ATX AMD Motherboard - AMD Motherboards

Newegg.com - SAPPHIRE 100195L Radeon X1650 512MB 128-bit GDDR2 PCI Express x16 Video Card - Desktop Graphics / Video Cards

Newegg.com - KINGWIN ABT-450MM ATX 12V Ver.2.2 450W Power Supply 115/ 230 V CB, CE, FCC, cUL, RoHS - Power Supplies

Newegg.com - A-DATA 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Desktop Memory

Newegg.com - AMD Athlon 64 X2 4800+ Brisbane 2.5GHz 2 x 512KB L2 Cache Socket AM2 65W Dual-Core Processor - CPUs / Processors

Newegg.com - Microsoft Windows Vista Home Basic SP1 32-bit English 1pk DSP OEI DVD - Operating Systems (XP is discontinued this summer, so upgrading to vista is inevitable D= )

Total (w/o shipping)- $444.94

Hey there AW!
The user just wants a new motherboard, processor, & ram.
They'd like to keep everything else, including their AGP video card & PCI soundcard
As for Vista, yuck, I'd rather get XP (or if needed by citizen, 2000) on ebay
...I'm anti-vista btw...

Vista?Ok I installed Vista Ultimit a few days ago.Im finding it diffrent fromXP,much diffrent.

It has good new features and a great look.
It is slow to boot but once loged on it runs smooth and great.Updates take a bit of time to install unlike XP,but I like it and am growing used to it.

Do not be afraid to try new things.

People just need to learn how to use it to its full advantage.

I agree. I have been using vista nonstop for the past month, and im liking it more than i did before. It depends on the system. It runs on this laptop smoothly, fast, stable and easy to use. Its not so bad as it was on my other system lol. There arent many AGP motherboards out there. Beleive me, i looked to change the motherboard on my old Athlon XP computer, but the only one is the PCCHIPS board, which has a terrible reputation. you would have to pay ME to buy that board lol. He will be able to use his soundcard, and if he desires, his case as well. If he doesnt want to pay for graphics card, that motherboard has a fantastic integrated solution, that is good for mild gaming (this laptop has the same graphics core and i can play FSX, WoW, and Sims2 all smoothly at decent settings) So take $120 off the price. $320 is not a great deal for a normal use computer.

Plus I think you need a decent computer to run Vista.People only see minimum requirements as in 512 RAM.

This is just not good enough,once you start to install programms and you want to play high spec games,downloading etc.You need around 2 gig RAM to run it at any decent speed and get good quality game play.

If you do not add the extra RAM or install it on a half decent computer and it do not run like XP then you would of course have problems.

I like Vista.

To me vista is a waste of money since i will have to buy unnecessary amounts of speed just to run it as fast as the old oses, speed that i could use on more importantant processes. speed that is wasted on the fancy gui appearence, like 3d windows. this is a strictly professional pc that i only need for music and sound. thanks for all your help guys. these are great suggestions.