Both cache size and sort size affect memory usage, so you cannot maximize one without affecting the other. Keep in mind that cache size is allocated on postmaster startup, while sort size varies depending on the number of sorts being performed. Generally, cache size is more significant than sort size. However, certain queries that use ORDER BY, CREATE INDEX, or merge joins may see speedups with larger sort batch sizes.
Also, many operating systems limit how much shared memory can be allocated. Increasing this limit requires operating system-specific knowledge to either recompile or reconfigure the kernel. More information can be found in the POSTGRESQL administrators guide, http://developer.postgresql.org/docs/postgres/kernel-resources.html.
As a start for tuning, use 10% of RAM for cache size, and 2-4% for sort size if you have just a few big sessions, and much smaller if you have lots of small sessions. You can try increasing it to see if performance improves and if no swapping occurs. If the shared buffers are oversized, you waste overhead maintaining too many buffers, and it takes RAM that could be used by other processes and as additional kernel disk buffer cache.
A valuable server parameter is effective_cache_size. This parameter is used by the optimizer to estimate the size of the kernel's disk buffer cache. In kernels with a unified buffer cache, this value should be set to the average amount of unused RAM in the kernel because such kernels use unused RAM to cache recently accessed disk pages. On kernels with a fixed-sized disk buffer cache, this should be set to the size of your kernel buffer cache, typically 10% of RAM.