For Adobe CS5, you must have a Mac multicore Intel processor with OS 10.5.8 or 10.6 or 10.7.
There is no reason to buy a used Mac with Snow Leopard (10.6) unless you have software that runs only in 10.6 but not in 10.7, or if you are on a very limited budget and can't afford a new Mac. For instance, some of the older MS-Office suites won't run in 10.7 (some of the older versions require the "Rosetta" capability others have been discussing here). You mentioned only internet browsing and Adobe CS5 as the software you require. If that is really it, then I thiink you are best off with a new Mac computer with 10.7. It will take longer for you to fall behind "the latest" this way and you will be able to install updates (for the Mac OS and Adobe software) for a good while into the future.
It can be a little risky to by a used computer. It would be good to have someone run some hardware tests and check it over for you if you aren't too savvy about these things. If you check the cries for help on these forums, a lot of them start with "I got a used computer and now xxx doesn't work ..." One of the hardest things to sense with a used computer purchase is an intermittant type hardware problem that you might not see until you use it for a while (long after the seller has disappeared).
You can get a state of the art brand new iMac with the latest operating system for not much over $1100. It actually costs less than Adobe CS5. But I don't know your budget contraints.
We have used Adobe CS with iMacs of various vintages (going back 6 years or so) as my daughter is a photographer who does intensive photo processing and editting, including use of RAW files. The iMac platform is a great one for this work, the newer ones have the computer power required.