Flashing a new ROM is pretty easy, and the more you do it, the easier it gets.
I recommend downloading Rom Manager, so you can save your ROMs when you decide to try out new ones. Doing this will prevent you from having to set up your phone everytime you flash back to a ROM you've set up before.
Flashing 101:
1. Using Rom Manager, make a back-up of your stock ROM, so that you'll always be able to come back to it at a later date.
2. Download your chosen ROM to your computer.
3. Hook up your phone to your computer as a USB drive.
4. On your computer, open up your phone's drive and copy the ROM you want to use onto the drive.
5. Eject the phone usb drive from the computer.
6. Using Rom Manager, reboot the phone into RECOVERY.
7. Using the volume keys, scroll down to "wipe data/factory reset", and click on it using the power button or the home key. Confirm it by clicking on "Yes" on the next screen, and let it do it's thing.
8. When the menu comes back up, scroll down to "wipe cache partition", and select it. Let it do it's thing.
9. When the menu comes back up, scroll down to "advanced". Click on it, and click on "Wipe Dalvik Cache".
10. Click on "Go Back", and then choose "Install zip from sd card".
11. Click on "Choose zip from sd card."
12.Scroll to the ROM zip file you put on the sd card and choose it. Confirm the choice and let it load onto the phone.
13. When the menu comes back up, choose "Go Back", then "Reboot Now".
14. The phone will then boot with the new ROM and you'll have to follow the steps to set up your phone like you did when you first got it.
As far as which ROM to download, it really comes down to personal preference. Ask 10 people what their favorite ROM is and you'll probably get 7 different answers. There is no true ICS ROM yet, and probably won't be until Verizon releases a leak, but there are quite a few that look like ICS to some degree. As TankMech mentions, Bully Desensed is a quality ROM, and in my personal testing, it had the fastest Download data speeds of any of the ROMs I looked at (results may vary). However, I think sfk's CM7.2 ICS/Jelly Bean themed ROM looks the most like ICS, and it has very fast data speeds as well. LiquidSmooth 3.2 is also nice and has a good ICS look to it. Honestly, I'd recommend trying a few of them to see what you like best. You might even decide that you prefer a ROM with Sense 3.0 or 3.5. Just have fun with it!