If your FSB is only FSB266, what you are mainly missing, is
memory bandwidth. A Barton 3200+ (200x11=2200MHz) has FSB400,
and will give you more memory bandwidth.
This is what I used in mine - AthlonXP-M 2600+ mobile:
http://www.newegg.com/Product/Product.asp?Item=N82E16819103436
The advantage of these, is the multiplier is unlocked. Even so,
I run mine at 200x11, and get the function of a 3200+ processor,
for $95. I'm using 1.65V, but some people get away with less
volts. I use a Zalman 7000 for cooling, and the processor doesn't
really get that hot.
The toughest part of a FSB400 upgrade, is the RAM. The Nforce2
is picky about RAM, and I recommend buying some PC3200 CAS2
memory, if running at 200x11 or higher. I used some Ballistix
($94 each) but there are probably 7 or 8 other name brands that
would work equally well. You can certainly experiment with your
current RAM first, and if the board is misbehaving (fails
Prime95 torture test), then you can consider a RAM upgrade.
The toughest conditions are dual channel mode (slot 1&3 or 2&3),
while single channel mode (slot 1&2) gives less bandwidth, but
also gives less trouble.
You didn't say what you use the board for - but I suppose
if your budget is limited, no sense in contemplating a S939.
Paul