It really depends on what you use your computer for. Do you want to run first person shooters on the highest settings? Do you want to use both of your monitors now, or just one of them?
I'd say 5770 ($149 right now) if you want to get a good deal and play most games on high settings. The 5770 is already almost twice the power of your 9800GT though.. so it would be more than capable of handling your new monitor. ..
or the 5850($300) if you want awesome performance at the very highest settings on two monitors and aren't too worried about money. Its overkill.. but hey.. why not? (The 5830 at $250 is just a bad deal in terms of price/performance)
That coolermaster wouldnt be a good choice for crossfire, but should be able to handle the single 5850 just fine.
I'll buck the trend here and say the HD5850 is your best choice, my 5770 can struggle at 1920x1200 despite being fed by a i5 750.
Of course the HD5870 can be found for a few dollars under your $400 maximum;)
It really depends on how long you plan on keeping the current rig and the settings you want to apply: The more powerful the card you install now, the longer it will keep churning out decent framerates at high settings.
Thank you all for your replies! Planning on using just the new monitor for gaming. single- Based on the response, I guess nVidia is out of the question? Was originally thinking of a GTX 275/285. Also, if I were to get a 5850 or 5870, which brand should I go for? Currently leaning towards XFX or Sapphire.
Regarding the PSU, the entire system is about a year old. One concern is that the 36 Amps the PSU provides is not enough to power a 5870.
As long as you are just running one of them you will be fine with the coolermaster. Nvidia is lagging a bit in technology right now... Brand doesnt matter too much, XFX has a better warranty but the build quality is probably similar in Sapphire, Asus, or Gigabyte.
As you see below, the 5850 is similar to a GTX 285 and cheaper:
@ Macestun: No, Nvidia are not totally out of the question, it's your money after all!
If you want to go with a GTX285, then, by all means do so, it will run off your existing PSU providing it has two connectors and its performance is comparable with the HD5850.
We are suggesting ATI cards because the current Nvidia cards lack DX11, burn more power and are more expensive for the performance they offer, although you may vey well find a good deal on one if you shop around.
As for brand, read the Newegg reviews (even if you do not live in the USA). If you see an issue that is reported frequently then you know this is a product to avoid.
If you live in the vibrant USA or beautiful Canada then go for XFX or BFG, I hear nothing but good words for their support and warranties, otherwise, just look for the best deals.
Do not pay more for non standard coolers for the ATI cards, the stock units do such a good job it's not worth the extra (in my opinion).