A 2600+ would either be a Sempron now, or an older Barton core. Bartons would be much better because of the larger L2 cache which some say would be much more responsive than the 256 kb L2 in Semprons. Bartons, however, run hotter than comparable Thoroughbred cores. If you choose a Barton, be prepared to bump up your HSF. While I personally would prefer a Barton over a Sempron, most Barton cores are now second hand and used previously with gaming machines so these usually take a beating.
I used to have a 1700+ Thoroughbred then I upped to a 2600+ Sempron. The amount of plugins I could run increased only by a few despite the speed jump. Tracking with the old Athlon was comparable to the new Sempron. At times, I felt that the 1700+ was more responsive compared to the 2600+ Sempron even though Semprons are practically renamed Athlon XPs. If you need to upgrade, get a Barton core but don't expect great increases in performance over your existing 2000+. Before anything, try overclocking your existing cpu, it might be more effective. Either way, you will require a much higher performance HSF since the Barton runs hot. One more thing, Semprons and Bartons usually run at a FSB of 333 MHz, make sure your motherboard can run stable at that speed; 3200+ Bartons have an FSB of 400 MHz.
Increasing ram is usually a good idea. It's usually recommended to keep your system cache at 1.5 times your installed ram. Remember to defrag your drives often because it's usually the hard disks that are a bottleneck in a recording pc. Better yet, have a 2 drive DAW, one for the system and the other for audio data; partitioning is usually not recommended for audio disks.