You don't have to worry about using GPT until you go over 2TB. If you were to go over 2TB you would have to make it a GPT drive or windows would not see more than 2TB. Just a heads up for those wanting a 3TB drive or wanting to use more than 2TB total in a Raid array.
And no, there is no converting, you have to wipe/rebuild the drive/array to go from one to the other.
8 Pro 64bit
AMD FX 8350
Asus Sabertooth 990FX R2.0
24GB GSkill/Corsair 1866MHz
2x XFX 6870 1GB
SSD - OCZ V4 128GB/Kingston 120GB
HDD - 3TB/2x 750GB/500GB/250GB
Cooler Master HAF 932 Red
CM Hyper 212 EVO w/2x SickleFlow 120mm Red
Logitech X540 5.1 Surround
2X Acer 23" LED - Eyefinity: 3840x1080
Microsoft Sidewinder x4 KB
"The significant problems we face cannot be solved at the same level of thinking we were at when we created them."
- Albert Einstein