Do you have needles appropriate to working with worsted weight yarn already? That should give you a pretty good idea what size needles you want for working in the round, your gauge should be similar in the round or flat. I only have longer interchangeable or fixed circs because they're more versatile. 40" (or thereabout) circs are great for magic loop and doing 2 @ a time socks. You could use 2 circs if that's better for you. Check out the Advanced Techniques videos in the free videos available at the top of the screen.
I should never overestimate my basic arithmetic skills.