jrepp,
If you have a Northern Tools catalog, flip to the airtools section and you can very quickly size up what is available.
I agree with Bird on several points.
1. CFM is king. If your compressor is lower rated than the tool, you will constantly be underpowered. Think "pow-pow-pow, wait-wait-wait, pow-pow-pow" Never be happy and throwing away $$ on torque you won't get. Also consider the size of hose and connectors you use if getting a larger tool. Larger tools usually require larger hoses and connectors.
2. Most impact wrenches have a higher reverse torque than forward. This gives more oomph for removing stubborn bolts.
3. 1/2" or max 3/4" seems reasonable for what you have. Forget 1"- lots of $$ and high CFM in the larger units. Very specialized.
4. Anvil- This looks like a built-in extension. Helpful sometimes, but a good extension will work well. A cheap extension probably won't hold up long, just to note.
5. Oil, OIL, OIL. Did I mention Oil? /forums/images/graemlins/grin.gif If you don't oil it, you WILL ruin it. More on this if you want...
6. Proper PSI. Don't make a habit of using too much pressure. Use a good regulator, and as mentioned earlier, most are rated at 90psi. Going over will eventually ruin the seals. Bad seals= poor power.
7. You usually get what you pay for... Brandnames like Ingersoll-Rand (IR part #'s), Snapon, etc carry a sometimes inflated premium, but have great support and repairability and rebuildability.
Hope this helps.
-JC