[Mobility Radeon HD 4330] KWIN/Compiz RAM usage

fglrx version that I tried:
- fglrx-8.600
- fglrx-8.620
- fglrx-8.650 (Catalyst 9.9 - binary from AMD ATI) - Current

The current situation I am experiencing is with compositing enabled in KWIN/Compiz, the system physical memory usage was sometime 500MB RAM higher than a normal expected behaviours. Does KWIN/Compiz actually use up all the texture (512 VRAM) that it needs to borrow 500MB more from Physical RAM even though I don't run any extensive graphic rendering apps? Maybe fglrx (or my graphic card) doesn't always use SIDEPORT but mostly use UMA (which is ridiculous and undesirable since I have 512 MB of discrete VRAM). There is no option to change in BIOS as well.

Same set of running applications (KATE, firefox, pidgin, dolphin) with compositing enabled on different boot.
* Expected behaviour: X MB of RAM.
* Exhibited behaviour: sometimes X MB of RAM, sometimes X+500 MB of RAM (majority of the times).

My current laptop specifications HP Probook 4310s are as follow
- ATI Mobility Radeon HD 4330 (512 VRAM).
- 3GB DDR3 RAM.
- KDE 4.3.2 with KWIN and default config.
- Ubuntu Jaunty 9.04 + nobackfill patch + binary Catalyst 9.9.

$ fglrxinfo
display: :0.0 screen: 0
OpenGL vendor string: ATI Technologies Inc.
OpenGL renderer string: ATI Mobility Radeon HD 4300 Series
OpenGL version string: 2.1.8918

$ Xorg.0.log (part)
(--) fglrx(0): Video RAM: 524288 kByte, Type: DDR2
(II) fglrx(0): PCIE card detected
(--) fglrx(0): Using per-process page tables (PPPT) as GART.
(WW) fglrx(0): board is an unknown third party board, chipset is supported
(II) fglrx(0): Using adapter: 1:0.0.
(II) fglrx(0): [FB] MC range(MCFBBase = 0xf00000000, MCFBSize = 0x20000000)
(II) fglrx(0): [pcie] 2040832 kB allocated