The
sweet spot for performance/cost right now is the ~256GB drives. The ~512GB drives perform still better, but (except for the
Crucial M4 right now) they're still priced at a substantial premium, and the performance benefits drop off after 256GB-ish. The controllers are just optimized for that size this generation.
The biggest benefit in daily use from an SSD is the random access time improvements. Sequential reads and writes are what old spinning disks do best, so comparing sequential (peak) read/write speeds doesn't paint the full picture of performance.
When a SSD helps is when the system is doing many dissimilar things on disk at once. Spinning disks spend most of their time seeking and waiting for the platter to come round. SSDs can access any "sector" on the disk equally in speed, in any order. That's the difference that matters most, and that's why the startup performance is the easiest to see. But all multitasking and disk intensive operations are dramatically improved as well.
Since jumping on the bandwagon, I can say firmly that I agree. An SSD is the single most important upgrade you can get for your computer. The benefits you'll see would almost certainly outweigh any other component you can improve.
But, yes, if you can afford it, go for a 240-256GB drive. If not, then a ~128 will do much better than the little single-channel 64s. The only way I'd use a 64GB drive anymore would be as a Smart Response cache.