Hi, Miguel
this can't be true - or something is bad hardware.
1. once more: close all applications you see (no icon in the dock is hichlighted).
2. open Activity Monitor (in Utilities), click on CPU, make sure that "all processes" is selected. Which process takes more than say, 10 % of cpu?
3. which Users show up in this table? only you and the system (which is called "root" here)? if not, please report back with this info.
4. Now click on RSIZE (still in Act.Mon). Which processes occupy more than 500 MB?
5. Now click on "System Memory" - anything green left? if not,report here.
6. Now click on "Disk Usage". How much green is left? report it here.
7. now terminate Activity Monitor, and do a hardware test - look here for details:
http://support.apple.com/kb/HT1509. It could be that the additional RAM which you have is a problem.
please report the results of every step, so that we can narrow down your problem.
Once more: make sure that there is NO application running!
Hope this helps - and REPORT BACK
_ SOON !!!
Thomas