I would get the Vertex 2 over Agility 3. The Vertex 2 is a more stable Sandforce controller than the Sandforce 22xx. Also, the Vertex 2 will probably perform better on the SATA II controller (Sync vs ASync NAND).
As your other question... SATA III is backwards compatable with SATA II.
As well I have an OCZ Vertex 2 60GB SSD and have never had a single problem. Just make sure you optimize it properly with one of the many guides on the internet. This can free up a lot of space that you will no doubt fill up with apps/games/etc..
The only difference in Agility vs. Vertex is the nand ram that OCZ uses. The difference is very slight and the winner depends on the application, so just buy whichever is cheaper. As far as 2 vs. 3 goes, tecmo34 is right.
I'm from New Zealand so the dollar is slightly different, that adds up to be around $120NZD (SATA2) and I do have $55 dollar credit from the store I mentioned, so yeah it is about $25 cheaper, Thanks for that .