Hey ive got a question... 3870's in crossfire or a single 4870?

the 4870 mops the floor with the 3870 in CF in games. the 3870 in CF performance is just about the 4850's, if not slightly better. Therefore go fo the 4870.
Better perfomance and less power draw. You will also get the extra slot to add another one later.
Check this out http://www.tomshardware.co.uk/forum/page-254925_15_0.ht...
Driver improvements have improved the 48xx series perfomance, this is an old thread but it should give you the idea

It really depends, do you need more performance?
If you are happy with your current gaming performance, I would defiantly not upgrade yet.
Your pair of 3870's is about as powerful as a 4850, or about 20% slower than a single 4870.
If you are gaming below 1920x1200, I doubt you will see to much of an improvement.

As an alternative, consider replacing your aging x2 5000+ with a newer Phenom II.
The extra CPU power should not only give you an overall faster system but it will also help feed your GPU's data faster resulting in better gaming performance at lower resolutions.

The 4870 definitely better than 2 3870s. But do you want to spend the money, let your 3870s go waste for not that large a gain in performance?

well, im not have too much problems running stalker with DX10 but i really want to enable sun rays. and my framerate goes down to the 20s with those enabled. i game at 1680x1050 but occasionally bring my comp to the front room and play at 1080p.. and that makes my system beg for mercy. especially for Warhead. i would like to get the 1GB edition 4870. thats about how much money i could get for my 2 3870s on craigslist  
wow, thats pretty pathetic that my 3870s are slower than a single mainstream card 4850.
i am actually stuck between those upgrades actually! would a phenomII 720 decrease bottlenecked performance on the 3870s(id be overclocking the hell out of it as well)? because i sure know that my 5000+ does pretty badly.
thank you for your replys all!

yehh the 4770 isnt the type of performance im lookin for  

thanks though

A bit dated, and not HD 3870 XFired, but the HD 4870 is compared to the HD 3870 X2 (close enough I guess):

http://www.xbitlabs.com/articles/video/display/ati-rade...

thanks for the read jaguar! i decided to hang onto my 3870s till newer stuff comes out and 4870s are as cheap as my 3870s were  
im definately gonna put money towards a better CPU though. probably the 720 BE or 920 looks like a good buy to overclock  
thanks everyone for your opinions!