A Suntour Ultra 6 might be your best bet.
It would add one more gear and still fit the spacing on your wheel. Just looking, our chain looks a little tight to me. Has it been a while since you replaced it? I'm always surprised by how fast a chain can stretch when then affects how the bike shifts. A new chain makes a big difference. The usual rule of thumb is to wrap the chain around the two bigs without the RD, then add a link.
Here's a Suntour Ultra 6 that is similar in range to your existing 5 speed. Best of luck.
http://cgi.ebay.com/NOS-Suntour-Pro-...3286.m20.l1116
You also asked how big a rear sprocket can a Campy NR handle? I run a 29 tooth on my Continental. It's tight but it works.
