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.
Go to your nearest supermarket or newstand and leaf through CPU magazine and other hobbiest mags. (Can't think of the name of the other one I have used.)
I bought my last two motherboards, AGP graphic cards and memory from TigerDirect. They are really good about returns if you mess up and they are good about providing tech help online to be sure you get compatible components. Both of my motherboards are FIC and the graphic cards are Radeon. A good place to get memory besides TigerDirect is Crucial.com. I've bought a lot of memory there. They have a memory configurator to help you find the right type and they also have live online help at no charge.
With the minis, just make sure your current PS is up to snuff for the new mb and vid. You could just throw in 40 or 50 bucks for a cheap case w/ ps instead. You might make that difference back by being able to get a regular ATX mb. I use crucial's memory configurator, and once I get the exact model of memory I need, go look at newegg.com to check out their prices. They are fantastic, inexpensive and fast. I've never had a problem other than the occasional manufacturer defect, and then the RMA was no problem at all.
Your first problem is the AGP slot.
Discover PCI-E and try again. :-)
Also look into the new nForce 4 motherboards. As far as graphics cards...the new NV50 cards from NVIDIA are on their way and are supposed to be amazing.
now you got me thinking about a new one myself ...
try http://www.motherboards.org/ for good reviews of motherboards, videocards, etc.