Reliability: 2x1GB
Performance: N/A
Reliability: Many boards can't do 200MHz+ with 2 or more sticks per channel. Most can do 2 sticks, but sometimes you can't do the tight timings. If it's dual-channel, you're well off, as 4x512 is really 2x512 and 2x512. But then you've got nothing left. Registered RAM helps here, but I can't find a single modern consumer board with support for it. There are even boards w/ ECC support, but not registered RAM.
Performance: For overclocking, 2 sticks will likely be best, for a simple reason: each stick you add could bring your max speed down. Your max is the slowest maximum of all chips on all the sticks. However, if you're not going too far over the stated speed, then it shouldn't matter. If you aren't overclocking, it won't make a bit of difference.