As far as the memory usage goes, VISTA (I assume that's what you have) uses what is called prefetching.  I guess the way it works is that it takes programs that you use frequently and stores them in memory for easy access.  If a certain program is not used then it will release it and clear up memory.  Or, if you need memory it will do the same.

With 4 gigs, dont worry about it.  I have 4 on my 1720 and it does the same thing, and I never have had a problem.  

As far as the black screen and the "display drivers stopped responding,"  click here and download the newest driver rleased on the 1st of July.  It specifically addresses the black screen issue.  It does say: ISSUES FIXED Inspiron 1720 "Display goes black and comes back with error "nvlddmkm.dll stopped" but it also says that it's the same driver for the 1520, so it might help you also.

but i talked with Microsoft and they told me" in vista (with areo interface) ram usage about 256mb. and it cant be that much." then i talked with dell. call center told me to make bug test. i found one hardware error but in deep scan i cant find anything.

now im suspicious about my graph. card not workin properly by the day i bought my laptop..

so i dont wanna waste time in repair time in services, i want to find solution.

so i can change the broken parts by service quickly..