For RAM, I've always used Corsair XMS for my main systems and never had a single issue.
But, from what I've seen, the pqi TURBO DDR2 667
are also an excellent choice, not that they're better than Corsair XMS but for the price, they're an exceptional value.
They overclock sky-high, seen several people very pleased with those modules. For when I replace my 2nd system, I'm putting those 1GB modules in it, along with this 660 I have and use my 8000ULs for a 960.
But if you're torn between XMS2 and OCZ, most definitely XMS2, I don't think I'd concider OCZ for DDR2.