Suggestions for video card for nF590: 4770 or GTS250?

I would definitely upgrade your motherboard before you replace your video cards. Anything PCIe 2.0 video card running in a PCIe 1.0 slot on the motherboard, the Radeon 4770 will be running literally half of the speed it was intended, which in other words, you would be wasting money until you purchase a new motherboard with a PCIe 2.0 slot.

PCIe 1.0 has a data rate of 250 MB/s and a transfer rate of 2.5 GT/s. The PCIe 2.0 has a data rate of 500 MB/s and a tranfer rate of 5.0 GT/s.

You would be getting a little more performance than your previous video cards (7900GS), but it wouldn't be mind blowing. Maybe it'll blow your mind how much you spent on your new Radeon 4770 and how little the performance improved.  

Like you said, the bottleneck is the PCIe 1.0 slot on your motherboard. I would recommend upgrading to a motherboard with a PCIe 2.0 slot before upgrading your video card.

Have to disagree with the poster above. Those cards especially do not saturate the interface. 1.0 will not handicap your card/performance. THG did a test when the mobo manufactures/video card companies introduced 2.0. There was negligible difference.
http://www.tomshardware.com/reviews/crossfire-meets-pci...

You cpu will be the bottleneck, which is not uncommon. The 4770 is a great 110 dollar card. You have one 6 pin power plug for it.

A HD4770 or GTS250 will not get bottlenecked by the PCIe 1.x interface they are not powerful enough to use the added bandwidth of the PCIe 2.0
GTS250 is an evolution of the 8800GTX cards of the past that were PCIe 1.0 and provides similar performance.

+1 Rolli59: There is no reason to change the motherboard, PCI-E 1.0 is more than fast enough for the cards you have in mind.
The GTS250 is a little faster than the HD4770, but the ATI card draws less power and runs cooler. Both should be a nice match for your CPU.
With an Nvidia card already installed, I would suggest getting the GTS250 to avoid possible driver issues.
One last point: What is the resolution of your monitor? If you're playing at 1280x1024 the 9600GT might be a better match for your needs.

I actually have a 30" monitor and my desktop res is maxed at 2560x1600. I try to play my games at 1600x900, but often have to go lower for higher quality. My hope is to be able to have more room with gaming. Is there a particular "sweet spot" resolution for the cards I'm thinking about?

1980 x 1080 is a good resolution for gaming. With the upgrade in video card, you should be able to hit that nicely.

The 1gb on the GTS250 is better for that resolution. I am inclined to say though that the HD4870 is worth it specially if you can get some overclock on your CPU.

Sorry, follow-up question: how big a difference is 512MB vs 1GB? What are your thoughts on these two?

http://www.newegg.com/Product/ProductReview.aspx?Item=N...

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Thanks in advance!

If your monitors native res is 2560x1600 then it is 16:10 so you should try to stick to resolutions at that aspect ratio. Either of those cards are rather weak for 1920x1200, they should do acceptably with 1680x1050. A good idea would be spending a bit more for an HD5770 which should do decently at 1920x1200 and is DX11 compatible.

i would suggest u get the 4870 or 5770 1gb . at high resolution , like 25x16 , there is a difference in 512m and 1g memory . not the case in 1440x900 or 1280x1024 .

A 5770 would be good at the moment. A wee bit more future ready!  
The difference only comes into play at high resolution. If the price difference is small then go for the 1GB version!