Well as of right now, even with the 64bit OS's there arn't many apps at all that would fully use the whole 64bits as it was ment to be used
That being said, it would still be hella cool to have a dual opteron setup right now. Only problem is that you might find it a bit of an annoyance to find a Mobo that supports what you want to do. My suggestion is you buy a Single Athlon 64 3400+, Get the Matrox x.100 for capture (freaking amazing capture card), a boatload of ram 2+GB, and some wicked ass SATA 10000RPM Raid array. Just any video card, for now, and wait for the new Nvidia / ATI cards that are comming out this spring. Finally top it all off with an Apple HD 24" display or some other massive LCD. This should take a fair chunk out of ur cash, and give you have a fun toy to "take advantage of Cool New Technology"