If your MB has got to be more than $250, and your graphics card has got to be over $150, that leaves $50 for RAM. I wouldn't put anything less than a gig in a combo like the one you're envisioning, but that's going to run up your budget.
A gig of 266 memory runs about $120-150 so I was thinking of a MB/CPU around $190 and a AGP of about $100 - but of course could scrounge the extra $100 for memory, if the MB/AGP deal was worth it.