Memory upgrades, may make a small difference to performance, but this is likely to be negligible: http://www.readynas.com/kb/faq/performance/will_increasing_the_amount_of_memory_in_the_readynas_improve_performance
Memory upgrades are not supported and are not recommended. If your memory module is working fine I wouldn't replace it.
If you really want faster performance you should get a x86 ReadyNAS (e.g. Ultra, Pro). These have fast CPUs. Desktop x86 ReadyNAS come with 1GB RAM.
You can transfer your data across your network from a Sparc ReadyNAS (e.g. X6) to a x86 ReadyNAS.