You can use any 2.5" S-ATA HDD with a maximum height of 9.5mm, but know that the MacBook from 2006 has only an S-ATA 1.5Gbps interface, thus 148MB/s is the maximum speed for read and write actions.
It uses SATA 1.5Gb/s so any SATA SSD will work as SATA is backwards compatible. Assuming you use the HD bay of course, the ODD uses PATA which would require an adapter.
Just plopped a crucial M4 128GB in my 1st gen Macbook and so far so good. Enabled TRIM without any hiccups and it is nice and fast. That being said, the 1.5Gbps limit is a real bummer. On my PC w/ SATA III I'm getting 3 to 4 times the performance as I am on the macbook. I guess I can't expect much for how old this guy is. The performance improvement is very much worth it though even if I am still wary about the whole TRIM issue.