Memory Timings Problem

I have a question regarding memory timings with my Athlon 64.
I purchased two sticks of Mushkin PC3200 DDR SDRAM, timings 2.5-3-3. I use an MSI K8TM-ILSR MoBo with an Athlon 64 3400+ CPU (Newcastle). The MoBo has two DIMM slots, and states it can run up to 2GB memory at 400MHz.

If I only use one stick (512MB) - either one - the computer will boot up at 400MHz no problem. If I put in both, the computer boots up at 333MHz memory speed. I have run MemTest on both sticks independently and together, with no errors.
I knoe the S754 motherboards and the A64 CPU Memory Controoler can be finnicky with memory timings. These are identical sticks of memory. Is there anything I can do?

If not, which is better? 512MB of RAM at 400MHz, or 1GB of RAM at 333MHz? The heaviest application I use is Adobe Photoshop, sometimew with several layers and photos open at once.

Thanks for any help.

Edward

I have the latest BIOS, and my BIOS does not allow you to lock the memory timing, only "limit" it or "Auto". I have tried to limit it at DDR 400, but it still boots up at 333MHz.
I have even tried upping the memory voltage from 2.6 to 2.7V after talking with Mushkin, but no luck.

So once again, if I am SOL because it is a BIOS/MoBo issue, that is fine, but what would be better....512MB of 400MHZ or 1GB of 333MHz.

Check the rail outputs on your power supply. Make sure the 3.3v output is adequate and you have at least 17a on your 12v rail. Mushkin RAM and MSI boards don't like each other much. Mushkin recommends 2.8v for Vdimm. I fought the same problem and finally solved it by getting rid of the MSI board. Someone else has it now and it's running fine with OCZ memory.

Abit IS7 - 3.0C @ 3.6ghz - Mushkin PC4000 (2 X 512) - Sapphire 9800Pro - TT 420 watt Pure Power
Samsung 120gb ATA-100 - Maxtor 40gb ATA - 100
Sony DRU-510A - Yellowtail Merlot

Thanks crashman, that is what i thought. You always seem to know your sh*t, and I value your opinion. They are double-sided DIMM's, so it is indeed four banks. This seemed to a common problem on the AMD forums, but I wanted to make sure.

Let me ask you then, which is the better setup with a stock 200MHz HHT:

1 GB of 333MHz timed at 2-3-3-5 or
512MB og 400MHz timed at 2.5-3-3-7

Would love to know what you think