I can't for the life of me find the actualy speed of the 2800's, but I can remember the speed of my old 1800's - 1500MHz.
However, I upgraded from the 1800's to Opteron 246's and was blown away by the difference in single threaded applications, and that isn't even considering improvements in multi-threaded ones. (An animation that took 3 weeks on the 1800's ran in 3 days on my opterons. (Given, I have a different graphics card but it isn't a major upgrade from the one I rendered on for 3 weeks.)
Point being, technically I only upgraded from a total of 3.0 Ghz to 4.0 Ghz. However, the architecture of the chip is better (and 64bit capable), the bus speed is much faster (Hyper Transport!
, the memory is much faster(Integrated Memory Controller), the only thing that hasn't changed much is the graphics. Still AGP 8x and on a last-gen craphics card; and yet, I have a performance gain like that?
So, even if you are going with a slightly slower Ghz Opteron, you should still see a good performance gain overall. Or, if you want to upgrade your processor even more you can consider usint SATA hard drives instead of SCSI as they are now almost as fast and dependable but much less expensive per Gigabyte. (I'm assuming you are running SCSI in your current setups.)
Anyways, everything has improved across the board (no pun intended) since you bought your systems.
Ooh! I found it! 2.13 Ghz.
So, I'd say even with 246's (2.0 Ghz) you'd see some improvement due to the better architecture and higher bus speeds. However, to be sure, go with 248's as they are 2.2 Ghz processors. Either way you'll be happy. (And, they will be upgradable to dual core processors when you need to upgrade again in a year or two. You won't have to replace anything else even - unless you want to.)
Iwill Zmax DP SFF Barebones kit... (300W PSU, 8 USB, 1 FW, etc...)
Dual AMD Opteron 275's | 2 Gigs (2 Dimms 1024Mb) Patriot RAM 3-3-3-8
WD Caviar 80 Gig HDD - System
WD Raptor 74 Gig HDD
ATI Radeon 9800 Pro 256
NEC ND-3520A Dual Layer DVD±RW | Mitsumi Floppy/Card Reader combo