IMO, the best upgrade path here would be an ECS K7S5A with an AXP 1600-1800+. You could use your old RAM if it is PC133 capable. If not, get some good quality PC2100.
Lowest cost in above scenario:
~US$170 with old PC133 RAM
~US$230 with new 256MB DDR RAM
If you have a larger budget, well, tell us what it is.
I thought a thought, but the thought I thought wasn't the thought I thought I had thought.