Get some TCCD based stuff. Not very expensive considering that they can do 300MHz. PDP XBL is 229/1GB, PQI Turbo is $245 Other manufacturers like OCZ and Corsar hand pick their stuff and sell for around $270
Stock timings seem slow, but the EB performs.
I have both the PC3200 and the PC3700.
The PC3200 maxes out at 245Mhz, 3.1v at 3-2-2-11 timings on a Infinity. You can do 2.5-2-2-11 between 200 and 225. My new PC3700 reaches 260MHz, 3.1v so far at 3-2-2-6 timings
Waste! What is wrong with your current memory? It's BH-5. People kill for that and pay upwards to $360 for it. And there's no doubt it will overclock to PC4000 speeds and higher when given more voltage.
But to answer your question,
Yes, the PC4000 aka ddr500 will run in your ddr400...