Having all 4 dimm slots populated does put more stress on the NB, but you shouldn't have any problems unless you are on the bleeding edge of FSB speed. 500FSB should be easy, and that'll get you 4.5GHz (which is probably the most you can expect on moderate voltage...4.2-4.3GHz is more common for 1.365v).
If you only want 4GHz, and you know the RAM will do 450 then I don't see you having any issues. Having RAM that would do 500 would give you more room to play. Or you could get an E8500 E0 for the extra 1/2 multi, but personally, I'd replace the RAM before I replaced the CPU. From what I can tell the E0's (E8400, E8500, & E8600) are all pretty much equal on air/water. Its only when you have extreme cooling that the extra multi helps. These chips aren't FSB limited.
So, replacing the chip means spending more $ on a pretty much equal part. Replacing the RAM means spending more money on a higher quality product.