well for AMD64 under $300 is pretty difficult if you want the best 64 which is Socket 939. Socket 939 supports dual channel and will be the only socket AMD will continually update and support whereas they will drop the socket 754 and 940 and not really improve upon it....I think now they have socket 939 AMD64 3200+'s but the last time I checked it was the 3500+ which is $350 alone and $125 for a good motherboard.
Aright just did a little search, and at newegg they got a socket 939 3000+ 64 CPU which is $169 but it's OEM so you don't get manufacturer warrenty nor do you get a heatsink and fan. Above that is the 3200+ which is $199 also OEM, Retail 3200+ 64 is $215.....so $215 plus a board such as the ABit AV8-3rd Eye or MSI Neo2-Platinum (both excellent boards) will cost you $125 for the board (both boards are the same price basically) and $215 for retail 3200+ 64bit CPU is $340 and thatway you have the manufacturer warrenty, or if you went with the 3000+ 64 then it'd be basically right on target for your $300 budget, but like I said will be OEM.
Thats the link for the Socket 939 AMD64 3200+ CPU Retail version for $215