As said, the command rate difference only accounts for a few percentage points. I havent personally seen any benchmarks to show whether this is a synthetic difference or actual FPS difference. If its synthetic, then FPS difference will be almost nothing though things like loading times may be SLIGHTLY affected. Conclusion is I'd get 4x512mb.
Also, Matthias99, why would you be forced to DDR333? Do some mobos have issues providing for all 4 DIMM slots filled?
As for me, I'm running 3x512mb, 1x256mb. So I'm single channel (64 bit), 2T, with AUTO timings. How's that for slow memory!! It'd be stupid to sell my old sticks for jack and pay $$ for some CAS 2.5 or less 2x1GB (with the extra 256mb difference aside). The money spent just isn't worth the difference, and the money would likely be better spent on other components to the system (tho I'm pretty set all-around). If anything I'll just give away the 256mb stick and buy a single 512mb, so I'll have 2GB in dual channel, but still 2T and auto/slow timings. Ah well!