Just bought an Asus A7N8X deluxe, Athlon 2200+, and 2-256 mb sticks of crucial PC2700 memory for $356 altogether. Loving the system.
Take it down to PC2100 memory (all you need anyhow if you don't plan to swap CPUs later), maybe shave a couple mhz off the cpu speed, or go down to just an A7N8X (not deluxe, if you already have a sound card and LAN card), and you'd be right in the $300 ballpark.
If you don't have a sound card/lan card, get the deluxe... it's $30 more for highly-rated sound and 2 lan ports.