Hi,
Marcin is right.
Beter is to have bigger cache than use memory space for ramdisk.
Even if you use ramdisk for tempdb still ASE thinks that devices are located on hard disk and still ASE needs a cache to read any page from tempdb. In that case you need use RAM for RAMDISK and for cache for tempdb pages.
From my practice if you create cache as big as tempdb it will be enough.
Exception is when you use in-mememory database. In that case ASE knows that all is in RAM so it does not need double caching.
Tomasz Romanowski