Hey Otis,
Interesting question...I just built two new systems: an A64 3200+ and an Athlon XP-M barton 2500+. The A64 was for me, the mobile barton for my son. I already had a 22" Diamondtron monitor for myself and a 19" Sony trinitron for my son.
I went all out with the A64 -- 1Gb OCZ 3700EB, radeon 9800 pro ultimate with zalman cooler, Enermax PS, Audigy 2, nice case, SATA drive, etc. I think I put nearly $1200 into this system. So, then my son saw how awesome my system ran games and his celeron 566@850 w/gefore3 ti-200 jsut did not cut it anymore. So, he had a $400 dollar budget with which we bought:
MSI K7N2 Delta-L $66 newegg
Mobile Barton 2500+ $88 newegg
2x256 Corsair VS $103 newegg
Thermalright alx-800 $20 svc.com
Sapphire 9800 128mb OEM $135 newegg
This card was a refurb from newegg...and get this, the card comes and it is a 9800 pro!
I had an spare Enermax 365 that I donated to him plus a generic 80mm fan for the heatsink; he reused his IBM ATA.100 hard drive and Asus CD-ROM, plus his existing boring, but function case.
So, figure about $100 for those spare parts and the cost comes to about $515 -- far less than what I paid for my system. So, we assembled the system, bumped his cpu to 11x200 @1.625 --- Barton 3200+ speeds.
Sure, my machine play 1280x1204 in bascially all games totally smooth (22" monitor), but with his 19" sony he usualy plays everything in 1024x768...and at the resolution his system is very fast! I certainly stopped for a moment and wondered why I spent so much!
So, if you are on a budget, I think OCing a mobile athlon is an excellent choice...
Cheers,
Dominic