According to the stats next to your name (thanks!), you have an iMac "Core i5" 2.5 21.5-Inch Aluminum (Mid-2011) model, which happens to be quad-core (helpful to your upgrading needs!). This model can hold up to 32GB of 1333 PC3-10600 RAM (also extremely helpful to your upgrading plans!). So you have an excellent machine to go forward with.
You don't say what version of Adobe Creative Suite you have, and that's the big potential showstopper here. If its CS6, you may be fine to upgrade to Yosemite. If it's CC, then you have no issues of course.
It seems like there are a lot of complaints about each of the OS versions up to the current one, so I'm very uncertain which one to upgrade to.
Yeah, a lot of people (even some around here) conveniently forget this. A lot.
As Harryb said before, I also am a long-time user of Yosemite (since the first beta) and was so impressed with the improvements and stability that I made it my main desktop OS since beta 3. There is one area I am having some issue with even in the current 10.10.2, which is with Bluetooth -- but I never did a clean install as Harry suggested. (I'm thinking about doing one in the near future just to see if it clears up the BT issue, which is actually rather minor but annoying).
Anyway, since you have no option for upgrading but Yosemite, I would strongly recommend a RAM upgrade to at minimum 8GB and preferably 16GB or more. It should go without saying that you should ensure that you have a LOT of free space available on the boot drive before doing anything, and of course a full backup (preferably a clone backup, but Time Machine is a-ok as well). You might also consider replacing the boot drive with an SSD if you don't need a huge amount of space -- SSDs keep getting cheaper and together with the RAM would make that iMac run 5x faster (no lie!) than it ever has before.
What you'd probably want to do (after installing more RAM and/or a drive upgrade, and making a full backup) is "buy" the free Yosemite upgrade, but instead of installing it them you'd use the free Diskmaker X to make a bootable 8GB USB thumb drive of the installer. Then boot from it and erase the boot drive, install Yosemite clean, then use the Migration Assistant to bring over all your apps and stuff from the backup.
Your Adobe apps may need to be reinstalled (this is normal), but apart from that you will have a nice new modern machine!