@viral, Do you have Game Center? Trying to fill up my list.
Yeah, there's a separate HD version of Plants vs. Zombies. Whether you want to buy it all over again is entirely up to you and whether you can justify your enjoyment of the game enough to play through it again.
Regarding RPGs, there's one little gem I found recently called Mage Gauntlet. Everything about the game oozes quality, and it has a great old school retro look to it.
Zenonia and Inotia are great RPGS. The later titles in f those franchise are "freemium" games, but they're perfectly playable without having to invest money into them.
Caligo Chaser is a great paid RPG, but it's not optimized for the iPad. Still great regardless.
On the MMO front, Pocket Legends and Star Legends are great freemium MMORPGs with tonnes of content, and Order and Chaos Online (not free) is essentially World of Warcraft for iDevices. The entry price is fair and the subscription fee to keep playing is minimal.
Also second what lulz said, all of Kairosoft's stuff is great, though I don't think any of their stuff has been optimized for the iPad.