I'm not sure what the upgrade price would be, but you can easily check it out at the Adobe store (link above). You do not need to install CS2, the CS5.5 installer will ask for your old serial number when installing.
You don't have to keep CS2 installed on your computer, you will use the serial number to install CS5.5.
You can upgrade to CS5.5 (still), soon you won't be able to, so do it quick:
Go to Adobe store and get an upgrade, price below:
There is no 64-bit or 32-bit suite, some of adobe applications run in both (Photoshop comes in both 32-bit and 64-bit), some are 64-bit only (Premiere Pro), so you just purchase Creative Suite and when you install it, you can install both versions of Photoshop (if you run 64-bit OS).
I hope this helps.
Thank you for the answer. It's really helpful.
One additional question: Can I later on upgrade an upgrade?
I mean: I have the original full CS2, now I buy the CS5.5 upgrade. Can I then later on upgrade CS5.5 to say CS6 and later CS7 and continue to buy upgrades to upgrades?
Yes, but don't you think it would be really weird if you couldn't?
One additional point: Based on currently published Adobe policy, you will NOT be able to get a discounted upgrade to Photoshop CS6 from anything prior to CS5. This represents a recent change.
So don't expect to be able to upgrade your CS5.5 directly to CS7. They really want you to buy every major version from now on.
And Adobe won't sell you an earlier version upgrade.
So the new policy makes the comment "Yes, but don't you think it would be really weird if you couldn't?" unweird