Multiple graphics cards

[zfactor]

I currently have a stock motherboard (I don't know what the exact specs are because it came with my system and I can't find any) with two ATI Radeon HD4850s (512MB ram each, not the 1GB ones).

If I upgraded my mobo to have 3 PCIx16 slots (the fast graphics card ones, I forgot the exact tech name...), and got a fast graphics card with three ports (like the Radeon HD4870x2, or maybe something less expensive), would the two cards I currently have (linked in crossifre)provide a noticable performance benifit? If I just got another 4850, would that be better performance for my money?

I can run all games maxed out, I'm just wondering what would be the least expensive upgrade option when I can't...

 

[RhombusHatesYou]

I can run all games maxed out, I'm just wondering what would be the least expensive upgrade option when I can't...

Not tri-fired 4850s. As rEvolution said, your best bet would be to drop a high end current gen GPU card in. Not only will you get comparable performance to your current setup but when/if DX11 features start becoming more widely spread you'll be able to use them. How happy will you be if graphics options start showing greyed out because your cards are all to old to handle them?

There's also the fact that GPUs are subject to the Law of Diminishing Returns - for each bridged GPU you add to a system the less efficiently the system handles each individual GPU as more processing power gets diverted to controlling the flow of parallel processed information (that's the really, really simplified explanation). That's on top of what rEv said about PCI-E lanes.

Unfortunately the power consumption and waste heat of GPU cards isn't subject to the same law, nor is price. Trifire and Quadfire (and their SLI equivalents) are for hardware whores not gamers, people who want to do unnatural things to benchmarking software rather than just have the best gaming experience they can.