I'm using the Cisco Software Advisor to try and find an IOS that will support my 2610 which is currently running v11.3. I entered all the info and I know how much Flash/DRAM I have but I'm confused about the 'Feature Set' option that you have to enter.
I'm not quite sure what feature set I need or what the differences are between them. The router is just going to sit as a border router connecting the PIX to the outside world. I am going to have two separate ISPs connected to it but won't be load balancing between them. I'll route specific traffic out each of them. Any help is appreciated.
As the name implies feature set tells you what features are contained in that software. To function as a perimeter router, as in your case, something basic like IP only feature set should be good enough.
To be sure that you are choosing the right IOS for your environment you can use Cisco Feature Navigator tool. Cisco FN lets you select the features you intend to use and lists the softwares that meets the requirment. You can then choose the image based on how much memory/flash you have.
Here's the Cisco FN link.