AM2 - FSB 800MHz mobo - will this work with FSB 1000MHz CPU?

Looking for a cheap way to reuse a good 7800GT AGP vid on a new AGP mobo with new Athlon 64x2 cpu. Don't want to buy anymore used mobos or antique cpus.
Found a $44 Biostar AGP board at newegg, says FSB 800MHz Hyper Transport (1600MT/s).
The Athlon 64x2 cpus of course are FSB 1000 / HT 2000 MHz or something like that.
What's the deal? If I buy for example a Brisbane 4000+ which I believe has a locked multiplier of 10, won't the cpu run about 20% slower than it should because of the slow FSB on the mobo?
The 4000+ is supposed to run at 2.1 GHz. If it is on a 800 Mhz mobo bus I would think the cpu would wind up running at only 1.68 GHz. Anybody know how this works?