Question about adding / mixing memory

I've just upgraded my CPU to a Q9650 and my OS to Windows 7 64bit. I'm currently running with 4GB of ram and would like to max out my board, which is 8GB, now that i'm running 64bit OS. I have a question about mixing RAM. I'm pretty confident with my findings during my research, but i'd like some input as to whether i'm correct or not. I see some people with similair posts saying its ok and some say not recommended.
currently i have an nForce 680i SLI mainboard and 4GB of RAM (see below):

---Motherboard---
EVGA 122-CK-NF68 version 2 (socket 775)
NVIDIA nForce 680i SLI
---RAM---
2x 2GB G.Skill Dual-Channel DDR2 @ 400MHz (5-5-5-15)
PC2-6400 (400 Mhz)
part#: F2-6400CL5-2GBPQ
SPD Ext: EPP

I'm looking at adding 4GB more of:
G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800MHz (PC2 6400) Dual Channel Kit Desktop Memory Model F2-6400CL5D-4GBPQ
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Both sets are "completely the same in specs" except for the frequency (400MHz vs 800Mhz). I should have no problems adding ("mixing") 2 sticks of the faster 800HMz with the existing RAM, correct? the only consequence would be that the newer RAM would run at the slower 400MHz?

Also, if i do mix these, i assume i should put the older 400MHz in slots 1 and 3 and the new 800MHz into slots 2 and 4, correct? or does that not matter since the newer sticks will be running at the slower speed?