By my invoice it's 667mhz
1 465-2379 Inspiron E1505, Intel Core Duo processor T2300 (2MB/1.66GHz/667MHz
1 320-4651 15.4 Inch TrueLife Wide-screenWXGA, for Inspiron 6400/E1505
1 311-5776 1GB, DDR2, 533MHz 2 Dimm for Inspiron 6400/E1505
1 320-4655 256MB ATI MOBILITY RADEON X1400 HyperMemory, for Insiron 6400/E1505
1 341-3109 100GB 5400RPM SATA Hard Drive for Inspiron 6400/E1505
I also have XP MCE installed and have NO plans on installing VISTA on this system. I'll also be at the maximum allow memory size of 2 gb's!
So I'm betting that the 667mhz CAS 5 stuff would be the best buy for my system. If it gets me the best speed at this cost than that is what I'll go with.
Edit: I forgot to mention that the manual states that I can have 667mHz DDR2 installed, but I know this doesn't always mean that it'll run at that speed.