Can I pair up these Dimm's without reducing performance?

[e_davis677]

I am currently running this system with Vista 32bit:

Asus P5W DH Deluxe
Core2 Duo E6750 2.66Ghz 1333Mhz FSB
2x 1GB 800 Mhz PC2-6400 128M x 64 64M x 8 5-5-5-15
ATI Sapphire HD2900XT 1GB DDR4
500GB SATAII Raid 0
Sound Blaster XTreme Music
Enermax 620Watt Crossfire Power Supply
Antec P180 Case

I purcahsed 2 more sticks of the same RAM but later realized that Vista32 will actually only give me access to around 3GB. So I now have access to 2x512MB PC2-5300U 667Mhz CL5 Dimm's that I want to install in place of the other 2 Dimm's I recently purchased (giving me 3GB) so I can use those in something else. Will this slow down my machine because technically my FSB is 1333Mhz so the 800Mhz on my 2 1GB Dimm's aren't really needed. Am I correct in assuming this?

Thanks

 

[The_OGS]

...my FSB is 1333Mhz so the 800Mhz on my 2 1GB Dimm's aren't really needed. Am I correct?

Yes, exactly.
With 333MHz coreclock on the E6750, you would require dual-channel memory @ 667DDR to equal the CPU bandwidth. PC2-5400 will do this, no problem
The advantage to having PC2-6400 is not that you can run it @ 800MHz, but that it will run nice and stable with low voltage and tight timings @ 667MHz...
So your rig would be fine with the 3GB, but would run only at the best timings the PC2-5400 will support.

2x512MB PC2-5300U 667Mhz CL5 Dimm's

These are really cheap - you can get these (in Corsair, CL4) in Toronto for ~$45 bucks.
Corsair actually calls them PC2-5400 because they are rated for 675MHz.
Anyway, synchronous 1:1 memory:FSB is perfect.
Memory bandwidth beyond that of the CPU is not overly useful, (although it can produce some impressive memory bandwidth benchmarks, heheh...)
Regards

 

[e_davis677]

Great, I really appreciate the quick response.

Thanks again.