Stability, your far less likely to run into problems running the 2 x 2Gb then you will with 4 x 1Gb. Some times you will need to tweek memory voltage to get the four dimm's to run stable. Plus then later if you should deside to move to a 64 bit operating system you still have two open slots to expand.
The difference actually isnt anything in terms of performance
2x2GB is better IF u want to add more of the same RAM later, 4x1GB is good if u want to keep it for a while
i myself have 4x1GB OCZ REAPER DDR2 800 RAM and i love them
You can get low latency/higher frequency 2x2GB kits as well so the "latency/frequency advantage" is a bad reason to buy 4x1GB IMO. 2x2GB is the obvious choice as it leaves room for a future upgrade. As Maziar said if you plan too keep 4x1GB until your next upgrade(motherboard) it's a good (maybe even cheaper) alternative.