Intel G41 graphics are slow while memory runs in dual channel mode.

[N V]

Hello.

I have Compaq 500B with "Pentium(R) Dual-Core  CPU      E5300  @ 2.60GHz (2600.02-MHz 686-class CPU)."
Video card is:
%pciconf -lv | grep -i g41
    device     = 'Intel G41 express graphics (PCIVEN_8086&DEV_2E32&SUBSYS_31031565&REV_033&115)'
%grep -i intel /var/log/Xorg.0.log | head
(--) PCI:*(0:0:2:0) 8086:2e32:103c:2a8c Intel Corporation 4 Series Chipset Integrated Graphics Controller rev 3, Mem @ 0xfe400000/4194304, 0xd0000000/268435456, I/O @ 0x0000dc00/8, BIOS @ 0x????????/65536
(II) LoadModule: "intel"
(II) Loading /usr/local/lib/xorg/modules/drivers/intel_drv.so
(II) Module intel: vendor="X.Org Foundation"
(II) intel: Driver for Intel Integrated Graphics Chipsets: i810,
        Mobile Intel® GM45 Express Chipset,
        Intel Integrated Graphics Device, G45/G43, Q45/Q43, G41
(==) intel(0): Depth 24, (--) framebuffer bpp 32
(==) intel(0): RGB weight 888

The system is:
%uname -mor
FreeBSD 8.2-PRERELEASE i386

There were 2GBs of memory running in single channel (one slot occupied). Everything was ok.
I've added 2 more GBs (occuping the second slot). Now memory runs in dual channel. I have 4GBs. The system is i386, non-PAE, so ~3GBs are visible.
I have latest KDE that is built from ports.
And the problem: after adding memory, graphics (compositing, I think) became so slow rendering the effects, that after a second or less of some "desktop effect" KDE turns off desktop effects. XvMC is enabled in xorg.conf, if it matters.

Regards,
Vans.