The 939 is probably your best bet for future upgradeability, however keep in mind things change very fast. Just because you buy a 939 motherboard right now, doesn't mean you will be able to take full advantage of later 939 chips. If you are planning on buying an "exspensive" CPU in short order your strategy may work out for you. Of course you are sort of throwing money away if you buy one CPU, then another 6 months later, just to save $100 up front.
Realistically I have always had to upgrade the motherboard with CPU. Basically becuase there is always some new feature that I wanted, wether it be faster FSB, supports better memory, etc. Then normally there is some new memory that would also be better.
Memory isn't the place to cut costs. Just buy brand name RAM, that has a CL of 2.5 instead of 2. That will save you a little and you will still have 90-95% of the performance. Also buying single chips to run in dual channel, normally will save you a little money over buying a double pack.
If you are really trying to stick to a tight budget, maybe a SocketA setup would be better? You could get a really good system, and wouldn't cost you near as much.
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