After a fresh reboot :
total       used       free     shared    buffers     cached
Mem:       1024416      88576     935840          0       4756      39852
-/+ buffers/cache:      43968     980448
Swap:       987956          0     987956
start X with nouveau driver (dual-head) and KDE as DE
             total       used       free     shared    buffers     cached
Mem:       1024416     773648     250768          0      13984     106368
-/+ buffers/cache:     653296     371120
Swap:       987956          0     987956
start X with nouveau driver (dual-head) with fluxbox
             total       used       free     shared    buffers     cached
Mem:       1024416     733908     290508          0      15484     117460
-/+ buffers/cache:     600964     423452
Swap:       987956          0     987956
kill X
             total       used       free     shared    buffers     cached
Mem:       1024416     184412     840004          0      15516     115172
-/+ buffers/cache:      53724     970692
Swap:       987956          0     987956
reboot and startx X with the blob with KDe as DE
             total       used       free     shared    buffers     cached
Mem:       1024416     301612     722804          0      13948     106580
-/+ buffers/cache:     181084     843332
Swap:       987956          0     987956

Then I try just to start X 

Before I start X
total       used       free     shared    buffers     cached
Mem:       1024416     274868     749548          0      26288     186392
-/+ buffers/cache:      62188     962228
Swap:       987956          0     987956
After I start X
             total       used       free     shared    buffers     cached
Mem:       1024416     809332     215084          0      26324     186372
-/+ buffers/cache:     596636     427780
Swap:       987956          0     987956