Good point about the brake calipers; forgot about those as all I've been thinking about recently is disks!
It sounds like you've got some good clearance so could potentially go a bit larger before you're restricted by the frame / brakes (the size probably being limited by the clearance at the rear). On the subject of the brakes it's also well worth considering how close the caliper is going to come to the tyres when the shoes / pads are on their wear limit.
How do you find the brakes? My road bike came with Tektros which I found to be somewhat lacking and were quickly replaced with 105 calipers. If you feel similar you might want to look at swapping the calipers first before committing to larger tyres.
Unfortunately I can't tell a lot about the rims from their model number alone; if you have the tyres off and measure their width between the hooks (the most out-board part of the rim under which the tyre bead seats)
this summary should give you an idea of suitable tyre sizes.
Sorry I can't help much more than this; too many variable and unknowns to be able to remotely suggest an appropriate size.
Again, what issues are you having with the current tyres that are encouraging you to change them?