Yeah, for gaming I would suggest a the <i> very minimun </i> a Radeon 9000 because it has DirectX 8.1 and you are on a budget. Yes you can definitely get by on a DX7 card, Heck I get decent framerates on the Radeon 7500C on my laptop. I doubt you will be able to fit in those motherboards into a Compaq case, lunchbox system companies manufacture cases that way so you will practically forced to remove what components you can from the old one. The only Chipset I know of on Athlons that supports DDR and SDRAM is the ALi Magik 1, and that chipset really sucks in performance. That is probably the chipset on the board that your looking at. What I suggest for you is an NForce board like the above mentioned one. On your motherboard, You can get by with the intregrated IGP 440 chip from NVidia for your video for a little while until you get some spare cash to upgrade to a faster video card.
My OS features preemptive multitasking, a fully interactive command line, & support for 640K of RAM!