Memtest errors when I use more than 2 sticks of ram

Hello everyone, my technical background isn't all that great so I'll try my best to explain the problem.

Recently I have experienced an increasing number of blue screens while putting the ram under any type of load. Naturally this lead me to believe that I managed to toast one or more of my 1 gig sticks in the 10~12 months I've had them (ordered 2 gig sticks and 2 more a few months later).

So my first thought was to test out each of them 1 at a time with a memtest program. So I took all but 1 out and ran memtest (4 times in all) and came up with no errors. My next step was to test each socket out to see if I had a problem with that and came up with no errors. My next step was to test two sticks of ram at the same time while switching out sockets... no errors. It seems my problem shows up when I install 3 or 4 of my sticks of ram. This leads me to believe that it's a possible power problem.

My problem here is I'm not all that great with BIOS settings and anything with most programs in general (I can do basics like build a pc and install windows and such... but that whole modify thing isn't my cup of tea).

When I run CPU-Z with 3 sticks of ram in it says I have 3072 MB and their running at 401.0~401.2 MHz
FSBRAM is a 1.2

I guess you'll need some basic info in order to help me out with this.

PC Layout:

Mobo: Asus M3A78-T
CPU: AMD Phenom x4 9950 Black Edition
Ram: G.Skill DDR2 1GB (PC2-6400) CL5-5-5-15 1.8V~2.0V (4 of these)
1 Seagate Barracuda 7200.11 1TB 3.5" Hard Drive SATA
1 Pioneer 20x DVD R SATA
Windows XP Pro 32bit.... I know I can only get like 3.25 gigs out of it >.>
500W turbolink ATX power supply
GeForce 9800 GT PCI-E 2.0

This is about all I have time to put up before work and all I can really think of anything else to add at the moment.

Thanks in advance for any insight to my problem (oh and with 3 sticks in I got a blue screen while typing this up... huzzah for Firefox!)