If you are planning to upgrade to the Athlon 64-X2, definitely. The X2 dissipates 110 watts a chip when running full blast (or 55 watts a core; AMD has implemented their Cool & Quiet throttling tech on them, so the clock speed and power consumption varies due to load). For a dualie board, that is 220 watts. The Nvidia 6600 series cards can pull -up to- 75 watts each, depending on the memory; the ones with the huge active cooling fans almost always have DDR-3 memory, which are hot and current hungry. Assuming those Chaintech cards are the most powerful, that’s 370 watts just for the video cards and two chip packages. You still have system ram, hard drives, the rest of the motherboard electronics, and a lot of parasitic resistance in the connectors. That 550 watt supply might be ‘sufficent’, but would be running extremely close to the limit, which is a sure guarantee of an unstable system. And as dependant as modern system components are getting on having a -stable- 12v power feed, you could get a host of symptoms that point to anything -but- the power supply as the culprit. Not to mention that the power supply would probably have a shortened life due to the heat and near constant max load.
Personally, I would recommend dropping the extra $$ and building all that into a Lian Li aluminum case, either the full tower or the caster server case. They are a joy to work in, they have the option of mounting plates for two supplies, all the case hardware is thumbscrew, and they usually throw in replacement faces for a floppy drive and a CD\DVD drive. Plus rolled edges on all the exposed metal, and enough 80mm fan mount points to actually keep the beast cool. I usually get my boxes at casedepot.com.