Problems upgrading RAM, 2 different manufacturers.

Hi all.

This is my problem.

Im trying to upgrade my RAM memory from 2GB to 4GB (I run Windows Vista 64)

My Motherboard, with a Core 2 Duo 6600 installed
Asus P5N32-SLI Premium

Old Ram installed
Kingston DDR2 800, 4-4-4-12, 2x1GB, 2v.

New Ram installed
A-Data DDR2 800, 4-4-4-12, 2x1GB, 2v.

Once I installed the second memory pair ( both kingston and Adata installed properly in motherboard slots so they can run in Dual Channel) I noticed windows vista started to run very slow. Even the window starting is now x2-x3 longer than before, and all applications I run in windows work very slow too. Some games are unplayable.

I tried each memory set independently, and both work very well at 800 setting, timing 4-4-4-12 and 2v.

With both memory sets installed I tried everything (I think) in BIOS.

-Tried at 800, with all timing in auto. Windows go slow.
-Tried at 800, forcing timing to 4-4-4-12, same thing.
-Tried at 800, forcing timing to 4-4-4-12, and forcing 2v. Same result.
-Tried at 667, all in auto. Windows slow again
- ... at 667, forcing timing to 5-5-5-15. Same.
- ... at 667, forcing timing to 5-5-5-15 and forcing 1.9v. Again Windows go very very slow.

Im about to give up and buy anpther set of A-Data memories, so if it is a compativility problem I will solve it.
But I still have the doubt that maybe is a motherboard problem, or maybe changing some setting in Bios I will solve that issue.

Any advice will be very welcome