Intermittent crashes with new build

I put together my first build a couple of months ago (the components are listed at the bottom of this message) and have been experiencing intermittent crashes every since. The crashes occur a couple of times per day and appear to be memory related (e.g., error code MEMORY_MANAGEMENT) and are not always the same. I've been trying to locate the source but so far without success. This is what I've done to date:

- Checked cpu and gpu temperatures at idle and under load. All are good (e.g., cpu was in low 30s under load).
- Reseated my memory sticks and ran memtest86+ overnight. There were no errors.
- Installed the latest gpu driver yesterday. Had one crash since.

I would appreciate some suggestions on what I should do next to debug this. Although running memtest86+ overnight probably doesn't rule out the memory conclusively, my guess is that the problem is driver related. Should I try reinstalling all drivers? Should I update the bios?

Also, I have not made any bios changes other than the boot-up sequence.

The Build
CPU: Intel Core 2 Quad Q6600 Kentsfield 2.4GHz LGA 775 Quad-Core Processor Model BX80562Q6600
Motherboard: GIGABYTE GA-EP45-DS3R LGA 775 Intel P45 ATX Intel Motherboard
Cooler: XIGMATEK HDT-S1283 120mm Rifle CPU Cooler, $36.99
Cooler Bracket: XIGMATEK ACK-I7751 Retention Bracket, $6.99
PSU: CORSAIR CMPSU-650TX 650W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Power Supply
Video: GIGABYTE GV-R485ZL-512H Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
RAM: mushkin 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model 996587
Hard Drive: Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM SATA 3.0Gb/s, $74.99
Optical: ASUS 20X DVD±R DVD Burner with LightScribe Black SATA Model DRW-2014L1T
Case: Antec Nine Hundred Black Steel ATX Mid Tower Computer Case
OS: Microsoft Windows Vista Home Premium SP1 64-bit English for System Builders
Misc: ARCTIC COOLING MX-2 Thermal Compound

If you didn't, reset your BIOS to defaults. This will rule out an innapropriate setting being in there, like trying to run the RAM at too fast a command rate or something.
(oh, then go back and change your boot sequence).

Resetting the bios didn't help - not surprising given that I hadn't changed anything other than the boot order. And I never had superfetch, prefetch, uac, etc. enabled.

Could Gigabyte's EnergySaver be the source of the problem? I've noticed that GSvr.exe always crashes. As a trial I've removed ES from the system. I've also removed AVG and gone back to Avast, which I've always liked.