You should not have any issues with compatibility; it should just work.
You're right the SATA 6Gbps does not change any of the drive's performance. Also, the cache size is not that important than many consumers think; certainly not as important as the CPU cache.
The biggest performance indicators are data density (GB-per-platter) and rotational speed (rpm). Those two statistics will tell to what generation/class the disk belongs. The latest gen are 500GB-per-platter; while your 640GB drive is probably 320GB-per-platter so one generation below that of the latest disks.
So if you want the fastest HDD; try picking a 500GB-per-platter disk. These come in 500GB/1000GB/1500GB/2000GB capacities. So a 640GB disk has to use different size platters: either 320GB or 160GB platters; i'm pretty sure it has 2x320GB platters, creating 640GB.
One 500GB-per-platter drive is the Samsung F3 500GB series. These are normal SATA 3Gbps drives though; and i suggest you don't look at SATA 6Gbps unless you're considering an SSD. SSDs that will come out near christmas this year will make use of the additional bandwidth offered by SATA 6Gbps.