My $.02:
I would rather have 2 x 1GB sticks rather than 4 x 512MB sticks. The main reason isn't highly technical--I'd rather have a couple memory slots open to upgrade if I ever need it especially since it seems 1GB system RAM will soon not be enough to run anything you want. I DID take into account the following caveats:
Smaller RAM modules are often "quicker" than larger ones--the memory controller has an easier time accessing the entire module.
Less compatibility issues when using smaller sticks.
Generally, smaller sticks are easier to overclock.
Larger sticks sometimes will not run at their rated speed/timings while an identically rated smaller stick will.
As for the Value RAM Vs. Premium RAM issue, I like to buy quality RAM, which can fall into either category. IMO, memory quality is one of the three most important things is building a stable computer, along with a steady PSU and quality motherboard. If you've picked your CPU and motherboard chipset already, check out some enthusiast forums to see what RAM works well with them. Also remember that the larger the stick, the more important it is to buy a quality part.
Speed & Timings: fast, low-latency RAM is nice, but it isn't the most important thing in the world. I'm running 2 x 1GB OCZ sticks in dual channel mode at Cas2-3-2-5 1T on an NF4 with a Venice core CPU. It's great, but I cannot tell the difference between them and 2GB of Corsair ValueSelect running at Cas3, except that I had to relax the Corsair's timings to get it to run stable.