Like kwickone said, you can use one of these App Store apps to free up memory. But I'd recommend you memtool, which is free and can be found in cydia.
Additionally or instead of these, you should use SBSettings and its Processes toggle to kill individual processes, without the risk of messing with system processes, cos it won't let you

Those can also be found in Cydia.
I'd also recommend searching this forum for threads about memory usage. You'll find a lot of useful information on this topic.
Edit:
I forgot to mention that Memtool also has the ability to terminate individual processes, so it's up to you if you use the processes toggle in SBSettings or not. SBSettings definitly has the quicker access and can be used within any other app, which is a big plus (I use both by the way; Memtool to clean up; SBS to kill processes)
Anyway, bottom line, if you don't already got it, you should use SBSettings, apart from the whole memory issue!