I've tried a lot of different SSD's over the years and I've been most impressed with Intel drives. The Sandforce chipset has gotten a lot of popularity for performance, but it's reliability is horrible. I've owned 4 OCZ SSD's that were all based on SandForce and all of them BSD'ed on me after 2 weeks. Even with updated firmware.
I wouldn't sacrifice reliability for a marginal performance boost. That being said, you really only have 2 choices. The 320 is a great value, but closer to SATA 2.0 speeds. If you want to keep the pace with the fastest SSD's on the market, you'll have to go to the 510 at 120GB which is significantly faster at SATA III speeds.
The fastest is the 256GB 510 which runs at near SandForce speeds.
So the best value is the 120GB 510. It doubles your 120GB 320 speeds but with only a 25% price increase and still hovers around your R2000 limit (probably slightly over). Just make sure you have SATA III waiting for it.