I have this server, Dual Intel Xeon 2.83GHz w 4gb ram. My shared buffers were set to 1756 MB.
We have a high traffic e-commerce site, yesterday like 80000 uniques. We cache most of our dynamic pages to disk so its wierd we are experience really slow response times with postgresql.
Usually the number of simultaneous connections i monitor is around 8 to 10, but yesterday night, maybe there was a spike in the traffic, it was going up to 100, so the pages were taking like 15 seconds to render.
As far as i know the default for max_connections is "100" on many Linux Distros. This would explain why your web-app is slow when reaching this limit.