Windows7 Freezes at Start up with too much RAM

Two days ago, I upgraded my winxp to win7. All the hardware stayed the same and everything works now, except I can only use a maximum of 6g RAM.
With 2 sets of two different brands of 2 gigs RAM, all of the RAM installed (correctly?), the start up freezes...even the start up fix freezes. When I take out just one RAM bar, the system works fine.
It doesn't matter which RAM I take out or which slot is empty, so it's not a faulty RAM or bad slot.
System works with 2gRAM, 4gRAM and 6gRAM....just not 8.

In xp, I had all the RAM installed. Even though the OS wasn't accessing it, the machine started up and worked fine.

Any ideas?

[QUOTE=that sounds like a recipe for problems [/QUOTE]

2 strips from 1 company = fine
1 strip from 1 company, 1 from the other = don't know
2 strips from 1 company, 1 strip from the another = fine
2 strips from both = not fine

I think Jack Sparrow is probably right...I'll check on it and report back.

Cheers

Windows7 64bit
Mobo= Asus P5N-E SLI, which supports upto 8gb RAM...
Memtest seems kinda drastic and time consuming. But then again, its my machine.
Perhaps I do need to keep the same brand of RAM for the full 8gb, even though it works fine to mix for 6gb.

Further thoughts?

crumblydonut,
Are the chips on the list of chips approved by ASUS?

Personally, I never, never mix memory chips. All the same manufacturer, all the same speed, etc.

These are both well known manufacturers.
And...oh...geez...: 2 of the sticks are actually 1gb. Ach. PNY is the manufacturer.
The other 2 are:
OCZ Platinum 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory

The OCZ's are both inside, the PNY has only 1.

So perhaps the different manufacturers AND the different sizes might lock me up if I use them all?
[even though 1 stick works fine]

Thanks for the responses.

I have read once on ASUS forum about problems using all the memory slots and it was stated that the memory voltage would drop just a little and cause problems. I would first try the Memory86 test and if it shows problems try increasing the memory voltage just a little. Also try the CPU_Z program as DocBrown mentioned to look at all the settings. If you can't get it to work, I would just run with the 4GB OCZ which should be ample. I have 4GB in my system and have never seen it use more than 2GB.

Jim