I have the same setup and I've chosen to leave it with the 300A@450. Since I don't use it as a primary system anymore, the price/performance curve isn't noticable over a network connection.
The 1.0 board is the most limited in CPU/BIOS support, so start by checking Abit's site for the latest BIOS and what CPUs it supports. Then you will have fun trying to find vendors that still sell something for that board. You'll obviously have to get a slocket (slot-socket adapter). I'll make you search a little easier by pointing you
here. They are an
extremely reliable company that has been around longer than the web.