For memory try crucial if you want a memory upgrade I have used Crucial, Microdirect and ocz - each many a time for different things - I usually order what is cheapest off which.
A handy feature that crucial and ocz have on their websites are utilities that you can use to identify which type of memory your system supports so you do not order the incorrect type.
Personally I dont think its so much a memory issue as a network issue - you may find that if you put your network card in half duplex mode it will help with this problem (involves delving into the network adapter settings). Also within skype (I am not a user so I do not know but I do know VoIP well) do you have the option to switch off compression and / or silence suppression as both of these can cause the clipping that your encountering.
If you want to do a keyword search on the skype forums you are looking for ways of resolving "Clipping" - this is problem of one of the following - packets arriving out of order (If you can switch from UDP to TCP that may help) - Switching off silence suppression may help (The VoIP software trying not to transmit silence to save bandwidth and not identifying very well silence/voice - especially prevelant in a noisey environment) and Switching from Full-Duplex to Half Duplex on the network card may help if it is a not so good network adapter (Handled in software rather than hardware). If you suspect it is memory/processing power kill all other apps you are running and run it dedicated to skype for a call and check the difference - this tho may be masking the full/half duplex issue due to cpu load.
There are so many avenues this could be - I would suggest to try the above first and read up on the forums about clipping issues.
Cheers
Jof
p.s. Other things to look at if you can get into the nitty gritty of things is the sample size transmitted - for uncompressed voice a smaller size is better but for compressed voice a larger sample size is better. But I am not so sure you can go into codec/sample size detail with skype as it is not SIP Compliant.