Seagate makes a 2.5" 7200 rpm 160 GB drive which performs well. See this test:http://www.barefeats.com/hard90.html
The difference between SATA 150 and 300 will not affect performance in a portable computer. Sustained data transfer from even the fastest disk is considerably less than SATA 150 speed. SATA 300 matters in a RAID or with port multiplier technology, but not for a single drive.
If you are getting a card adapter for an external drive make sure it is Firewire 800 or SATA. Otherwise your external drive will be slower than your internal drive and not much use as scratch space for PS. Also, partition the external drive into two partitions. For maximum performance, make the first partition (the outer portion of the disk) your scratch space.