Well I've been reading here for awhile and kept putting the project off since I haven't had the money to buy anything. I have some time and money now to put into the project

So I'm ordering the 7" Liliput LCD, and a 4 channel amp. I'm just trying to decide on the PC parts now. I have a lot of stuff lying around I could probably use instead of buying all new parts.
This is what I have:
Compaq Laptop
1Ghz AMD Duron
384MB RAM
20GB HDD (wont be enough for my music, I was thinking of maybe getting an external)
Desktop
300Mhz PII
256MB RAM
60GB HDD
My main use for the carpc would be music and gps. I dont think the laptop would be a good route because of the small hard drive, and powering on issues, and the 300Mhz desktop isn't enough power. I'm not sure if a 90W or 150W PS would be enough to power it.
I have around $300-$400 to spend on the PC part. I dont know if I should go for one of the custom built ones, or I've been reading suggestions for Shuttles. I'm not sure which route to go. Any suggestions would be greatly appreciated.
Thank You

Matt