the 2800+ tells you that forever
2800+ = approx 2.8GHz Intel
Ak - That 250gb vs the 150gb is referring to the chipset and it's ability to transfer data to and fro I believe. I just read something on this the other day, so I can't give you an absolute definitive answer, but this was mainly talking about the Socket 754's so in that scenario you should get the 250gb because if I remember right, it has a more optimal 'bandwidth'
According to MSI's website though they have both the platinum and FSR with the Nforce3 250gb
Personally I think you're just somewhat eager to jump into AMD64 bit.
Give them the socket 754, save your money up so you can get like a 3200+ 90nm and good socket 939 board. It'll be worth it in the long run IMO