An AMD64 system should work fine as long as it doesn't have the VIA chipset. If you want to get a great system at more of a price, i would recommend getting an Athlon 64 fx-51 (i would get refurbished due to its lesser price, if you have the 750 bucks to get one new, then i would also recommend that). Also, for athlon fx, get an SK8N, that is what i have, and it works great. I had some memory problems when mixing CAS types and brand types, but after a quick bios update, that error went away.
In terms of an amd64 3000+ (more or lesser as well), i am not sure that i would go with one at this time. All of the programs are optimized to work with Pentium 4/Xeon HT technology, and at the current time, you can get a P4 3.06ghz with 800mhz front side bus for around 205 bucks. That is cheaper than 5 of the athlon 64s (including the fx series). With a P4, you will not be disatisfied. I am an AMD fanboy (since i got my setup from them in a contest ), but to tell you the truth, you would probably be best off with a p4 system.
If you are looking for a program that is optimized for 64-bit processors, you will be looking for about another year or so. It was recently announced that Windows XP 64-bit edition will not be released until at least 4th quarter of this year. On top of that, not every company thinks that 64-bit technology is worth looking into right now. Mainly the biggest industry that is looking at 64-bit technology at the time is PC gaming. I have yet to hear any press releases from Avid, Adobe, Pinnacle, or any of the major companies, stating how awesome they think 64-bit is. I think that if you want the best machine you can have for now for video editing, but want to not spend a whole lot, you should go with the p4. If you have money burning a hole in your pocket, get an Athlon 64fx or the Athlon 64 3400+.