Man, what a dilemma :Q
You want top performance on a board that ONLY offers you a 100MHz or 66MHz FSB.
Let's start where you're at now...
You've got a PPGA 400, that may or may not go 600MHz on your MB. If your sloket has core voltage adjustments, you could try and jack that 400 up on a 100MHz FSB.
1. Does your sloket support coppermines & voltage adjustments? Reguardless of how you upgrade, a GOOD sloket would be money well spent. You can get an Asus for about $15, I think.
2. Do you have great cooling on it? A GOOD heatsink/fan combo would also be money well spent. You can always sell it or move it over to another CPU. I think you can pick up one of those Golden Orb things for about $15.
3. Basically, with a GOOD sloket and a GOOD heatsink/fan combo, your 400 will either do the 600 or it won't. But for an outlay of about $30, you've got a GOOD base to start from. I'd say try and push that 400 as far as you can. And save up more cash for a MB, Video card, Sound card. Plan now, for the long haul.