|You need to run the numbers to give you a better idea of what's going on.|
Since this is a P4, ideally you should be running a CPU:DRAM frequency ratio of 3:4. That would mean CPU at 200MHz (800MHz FSB), RAM at 266MHz (533MHz DDR). 200:266 = 3:4.
With DDR333 RAM, you're running the ratio at 200:166 = 6:5....with DDR400, the ratio would be 200:200 = 1:1.
The CPU bandwidth at 800MHz FSB is 6400MB/sec (800 x 8). The RAM throughput at 333MHz is only 2666MB/sec (333 x 8) so the RAM is the bottleneck. If you enable dual channel mode, the RAM throughput doubles to 5333MB/sec (333 x 8 x 2). That's better, but it's still bottlenecking.
The throughput of DDR400 in dual channel mode is 400 x 8 x 2 = 6400MB/sec. Obviously better but still not ideal.