The clock speed determines how fast your memory will be working... so unless you overclock your proc, you will be 'underclocking' your RAM if they are mismatched.

The few benchmarks I have seen indicate that dual channel/paired DDRSDRAM kicks single channel out of the competition. The only reason you'll want to buy dual-channel is if it's cheaper than two modules half as big. The number of remaining mem expansion slots might factor in too.

Personally, for the cost, I would just run 2 256MB DDRSDRAM PC3200 at the lowest latency I could find.

What is 'enhanced latency'? Is it actually a lower latency or just marketing mumbo-jumbo?