SLI with an i5 on a P55 platform?

Why not go for another HD 4870, which will give you approximately GTX 480 performance for another $150 (I think that's how much they cost). Besides, GTX 275 in SLI costs the same as a GTX 480 lol. A better option would be an HD 5850 or GTX 470, and no, two 8x PCI-e 2.0 lanes will not affect the performance greatly of most cards, apart from the HD 5970 I'm supposing.

It is not a waste on a P55, it is however a waste to buy two cards of old technology on a good new system.

Either pay the money for a 5850 which outperforms all GTX cards at 1920 x 1080 including the GTX295 which also leaves you able to Xfire in the future.

Or save money and just buy a single 4870 to Xfire with your current one which will outperform even the 5870 in plenty of games

Buy another HD 4870 and run them in crossfire. I have i7-860 - H55 and am running 2 HD 4850s in xfire.

Don't ! Send it to me !

You really need to consider that running two cards over a single GPU solution is going to draw more power and generate more heat than a single GPU solution. Now you said that the 5870 is a bit too expensive, so why not go with a 5850. The performance is top notch at stock and the reference model versions have been known to overclock to amazing heights. I'm personally getting an ASUS DirectCU 5850 for it's combination of cooler and overclock potential. Quite a few reviewers reported performance as good as or close to a 5870 once the card was overclocked. It's tag is considerably cheaper than a 5870 as well, which is nice for someone as budget conscious as you seem to be.

My rig is also based around an i5-750 and a P55 chipset, so I can definitely relate to your question. It was one that had me researching GPUs for days on end before finally settling down on one.

Yes, as some say here the 5850 is definitely the card I've been thinking about buying the most.
But I can buy two HD5770 for the same price as one 5850.. So which ones of those solutions are the best?
Spec wise the two cards are almost the same and in most cases the 5770 is faster than the 5850.
Here is an example of two MSI cards :

[MSI R5770-PM2D1G] <--5770
Clock 850 MHz
1 GB - GDDR5 SDRAM
RAMDAC 400 MHz
Mem Clock 4.8 GHz

[MSI R5850-PM2D1G] Ati 5850
Clock 725 MHz (the slowest of the two)
1 GB - GDDR5 SDRAM (same, albeit with 256 bit interface compared with the 128 on the 5770)
RAMDAC 400 MHz (same speed as the 5770)
Mem Clock 4 GHz (slowest of the two)

Now, put aside the 128 vs 256 interface, the 5770 wins on EVERY spec.
Both supports DirectX 11, OpenCL, OpenGL 3.2. So why would I pay twice the price for a 5850 when I can simply buy one 5770 for half the price of buy, or two for the same price?

Also, I think its a shame that ATi cards doesnt support PhysX, because in games like Batman Arkham Asylum and Mafia 2, the game will look uber sweet with PhysX enabled.. But if you enable it on an ATi card, the game becomes unplayable.

I recommend going with the 5850 solely out of preference. I believe the single card solution will run cooler, quieter, and with less power required than running two 5770s in crossfire. Another reason is my lack of faith in crossfire as proper solution to increasing performance. I too was faced with the same decision when I decided what card to go with, and the reviews I came across showed that each setup was better for different games. With the 2 5770s though I'm at the end of my upgrade stream while with the single 5850 I can still get another 5850 to improve performance down the road. Alternatively, selling a single 5850 down the road will be easier than selling 2 5770s.

Another point for me was overclocking performance. Overclocking one card is going to be much less taxing than overclocking 2. Furthermore I don't have to worry about a weaker card limiting the overclock performance of the other. This wasn't the most important point, but it definitely helped. I think the issue of upgrade path was ultimately the final decider as it gave my system a much better future proofing option in that I can always get a second 5850 or sell the one I have and upgrade to another better single card solution.

It would be silly to do Crossfire 5770's. Two 4870's is pretty close to 2 x 5770's anyhow!! It's probably cheaper just to pick up a second 4870 for Crossfire.

The 8x / 8x PCI lanes on a P55 will not reduce performance enough to make you care. Equipped with a i5-750 you'd be fine. Which i5 CPU do you actually have? Is it a dual core, or the real i5-750 quad core?   That can make a difference!

If you want a single card option, the ATI 5850 is definitely a great choice @ $309 last low price I saw. It will play anything @ 1920x1080 with high settings. It also has DX11 support, where as the GTX 295 does not. You'd have to jump to the GTX 470 to get DX11 support ($350).

If you really want to dump the ATI 4870, my wife could use it to replace her ATI x1950   She wants to play games again, and that old card just doesn't cut it these days!! She's got a Q6600 @ 3.2Ghz + 4GB RAM, so her system can handle it, just not that old GPU.

Don't do a single ATI 5770. It's basically the same performance as a single ATI 4870 is.

http://www.firingsquad.com/hardware/ati_radeon_5770_575...

It would only gain you DX11 support. Would be pretty pointless.

ATI will never support PhysX because nVidia has a patent on it.  

They doesnt have a patent on physics in a video game.. They just happen to own the PhysX technology.
If Nvidia had a patent on the Physics engine, Havok etc wouldnt excist.

But alas, since Nvidia owns PhysX it's doubtful we'll ever see physx on ATi powered PC's (unless of course you buy a dedicated PhysX card)

I went for an 5850. Ordered it today and its on in the postal system somewhere.

Many said here about getting another 4870, but the problem is that I have an 512Mb version and I cant find any of them on the market.
All I can find is the 1Gb version.
I've checked all the webshops here in Norway and even local resellers, but they mostly sell the 4870 at a price where I can get an OC 1Gb 5850 card.