The GeCube HD3850 I bought some time ago has reappeared on newegg's list again, but with quite a price drop this time. At the time when 3850 just came out on the market, I paid about $185 for my gecube 3850 512mb. Now its available for $130, since there are very little card out there thats worth $130, should I use this price drop opportunity and do a crossfire setup?
 
I'm running a DFI-P965-S board, it has CF support but only the main slot supports x16, second pci-e only works at 4x.
 
And how much performance gain will I get in games? Is it a good idea or am I just wasting my time?

Hmmm not sure about the effects of the other 4x slot, but I remember seeing price/performance on several card solutions including the 3850, and the only better one I remember was the 9600GT. So, if the 4x doesnt effect it too much, then go for it. Ill try to find that article, to see if it was 16x first slot and 4x the other

I'd  save your money and get a 4870/50.

Yep, even at $130, it's not very attractive, especially since it'll run at 4x. 3870 perform the same as a 8800gs/9600gt, but the 8800gs cost $99.99. Might as well wait for the 4870, to be released on the 31st of this month. A single one wouldn't be bottlenecked by that second 4x slot.

Heres the link, http://www.techreport.com/articles.x/14686 It was done  on a 16x16x mobo, sorry, cant help

 
But the HD4850 definetely won't be around $130. And I can't CF the 3850 with HD4000 series, or can I?

If it was a x8 or x16 slot it would look appealing since you already have the other card. but x4 is going to be an issue.  Save your $$$ or sell your current card for a better single solution.

I agree with Jay.  xfire isn't the best solution opposed to a newer single card but it still gives you decent gains here and there.  Since you are going to use a PCIe x4 slot though it will really hold back your performance to where it will be best to stick with a single slot setup.  
 
That I know about, no official statement has come out that says that R600 will xfirex with R700 (someone correct me if I am wrong, I haven't been keeping up lately).  Since they are based off the same architecture however, it looks like they might xfirex though.  It probably wouldn't be too favorable however since your R700 will downclock to the R600.  You might as well ebay your current GPU and go for a single R700 setup.  

very few games and applications benefit from crossfire setups mainly due to lack of optimizations and good, up-to-date drivers.
 
My advice: never due SLI/CF setups get 1 good card instead of 2 or more weak ones. Besides a lot of games still dont support or arent optimized for SLI/CF

Ebay and online selling won't be an option, I don't think I can get much for my current 3850, and the 3850 is not powerful enough to run the games at high resolution. So I either need to buy an $200+ single card, or spend like $130 to get a CF setup thats better than the single card option.
 
How much of a performance drop with secondary card running at x4 are we talking here?

I wont be surprised if you dont get any benefit.

Going from a single 3870 to two, in the best case scenario is only 50% faster and only in the most demanding game, Crysis.  And with your 3850 being a (slightly) crippled 3870, you can expect less than that.  That's pretty disappointing imho and not really worth the money.

I'd say wait for the new NVIDIA G200 and ATI 48xx line and just consider outright replacement if the 3850 isn't giving you the performance you want.

http://www.anandtech.com/showdoc.aspx?i=3209&p=5

PS: many benchmarks have shown, with today's hardware, there's no advantage of 4x over 16x PCI, so don't worry about it.  Don't expect these two new model lines coming out to change this either.

Check this review and decide.
 
http://www.thetechlounge.com/artic [...] Crossfire/
 
Maybe yes, maybe not.