Crossfire With Different VRAM Amounts

I built a PC for gaming back in March with the help of my nephew (as I hadn't been keeping up with PC hardware for a while). I initially ordered a Sapphire Radeon HD 6850-1GB which he said I could later get a second and crossfire for better performance. After ordering it I found that for $30 more I could buy the Sapphire Radeon HD 6850-2GB and switched to it, with full intentions of buying a second for crossfire later on. It appears that his card came out in Nov/Dec 2011 time frame so it's not exactly old.

I've been playing BF3, Skyrim and TERA and noticing some slowdowns occasionally, so I was going to add the second card for crossfire (Basically Max out what I can for better performance). When I went to Newegg.com, it shows that the 2GB version is deactivated and not likely to be stocked again. I have checked everywhere and only found one retailer who wants $199 and has a limited supply.

My question is: Is there any issues or drawbacks if I were to buy a second card but a Radeon 6850 1GB and use it for crossfire (therefore having one 2GB VRAM card and one 1GB VRAM card)? If there would be issues and necessary to have a second 2GB card (for $200), would I be better off upgrading to a Sapphire Radeon 7870 ($360) and selling my 6850-2GB (would about equal out in cost)?

Thanks.

Here are my PC specs:

CPU: AMD Phenom II X4-965 Black Edition
MOBO: AsRock 970 Extreme3
RAM: 8GB Corsair Vengeance DDR3 1600 PC3 12800
PSU: Seasonic X650 Gold
SSD: Corsair Force Series 120GB
HDD: Western Digital 500GB