There are plenty of benchmarks available.... try
http://www.barefeats.com for a start, they compare "old" G5s to the new Intel Macs. There really isn't any direct comparison between Dual G5s and the Mac Pros, though, it's mostly the Quad G5 vs. the Mac Pros.
This article though
http://www.anandtech.com/mac/showdoc.aspx?i=2816&p=1 does compare Dual G5s (even a 2.0!) to the then-new (Late 2k6) Mac Pros. Ignore the Photoshop benchmarks at the end.. that was pre-Universal Binary with CS2.
The latter article also illustrates how horrendously Apple has crippled the memory architecture of the Mac Pro with its awful FB-DIMMs. I think if anyone out there was using their heads they'd implement Dual-Channel DDR3-1600- which would A.) satisfy the 100% filled theoretical 1:1 bus:memory bandwidth B.) Cut down to two channels- easier for consumers, less expensive, and also more efficient- and, oh, yeah. C.) Be tremendously faster. And of course less expensive, since FBDIMMs cost too much, assuming unbuffered, non-ECC DDR3. Even dual-channel DDR2-800 would be faster than what they're doing now.
If app launches are a problem, that's related in large part to hard drive speed; in your case, your 250 GB drives are pretty fast, so you might want to look into optimizing them (meaning defragmenting, more or less). . . my guess is that your drives may be heavily fragmented. I just launched Photoshop CS2 in the same amount of time it takes you- only plugins being Alien Skin Image Doctor and Xenofex 2- but on a Beige G3 with a G4/500 card, 768MB RAM and a 120GB boot drive.... and that's accounting for the time it took two other hard drives to come out of sleep and spin up for use as scratch disks.
Actually, I *just* optimized the drive... it was severely fragmented.. and it had taken well over a minute to launch before. Of course, OS X launches things somewhat slowly anyway.. Photoshop 7.0.1 on OS 9 launches in about 17 seconds with about five plugin sets.
Mac OS X makes an absolute mess of our hard drives. My directory was 32% fragements (32%!).. and I've only had OS X installed on this partition for a couple months... and only ~15GB of stuff on it. I use DiskWarrior 4. Try that. You can also of course buy new drives for your current machine, and put them in a RAID array (may need to buy a card for that).
Also, assuming that Radeon x800 is PCI-express and compatable with the new Mac Pros, you['re going to want to use that instead of the 2600XT, as it's faster. Otherwise, you'll be gaming.. more slowly. Unless you don't care about 3D performance.
Hope this helps in some way.
