5670

At your resolution I believe the 512MB version will be fine.

1GB+ is generally recommended for 1920 x 1080 resolution or higher. While the HD 5670 is roughly 25% - 30% faster than the HD 4670, I don't believe you will notice much difference with the extra VRAM. I have only seen benchmarks for the 512MB version.

As for brand, XFX is the best because of the "Double Lifetime Warranty" in the US. EVGA also have a good rep. Never bought their cards. I have owned / currently own Sapphire, Power Color and Gigabyte cards. No issues on my end with them since the cards I owned never failed during the warranty period.

1. for this card i suggest you to get the one with 512mb ram. but if you play on higher resolution the extra RAM should help. btw the extra RAM will not make your game to gain more FPS
2. for dx11 card there is none. if you look into older generation 5670 performance is close to 9800GT but 9800GT should be a little better in term of overall performance. if you really into dx 11 games i suggest don't go any lower than 5770
3. i think the card should be fine on medium or even high setting if you only use 1280x720 to play you game. but if you go for 1600x900 you might have to mix the medium and low setting to get more performance in game (but still it depends on the games you're playing)
4. i take EVGA and XFX but i thought that EVGA only make nvidia cards to this day.

my current CPU is i3 2.93 OC to 3.3 GHz and MB is Gigabyte H55S2H.
is it necessary for the motherboard and Graphic card to have in the same brand or not relevent?
my old card is 4650 OC 650 MHz 1GB DDR 2, so if i buy 5670 512 MB DDR5 do i notice huge performance leap?
and how about 5570 compared with 5670?

i'll play (COD Modernwarfare, COD world at war, N4S undercover, RA 3, Counterstrike 1.8)

also World of Warcraft Dota

i'm sorry for my irritating questions.

i saw many reviews of graphic cards.it show the performance with FPS.

what should be minimum FPS to play a game and what's the optimal one?

thanks again

if i'm not mistaken 4650 performance is around Nvidia 9500GT. 9500GT to 9800GT is quite a jump so i think you should be able to notice the difference between 4650 and 5670. (well do correct me if i'm wrong on this one)

about the FPS it depends on the games. normally people consider 60FPS and above is excellent for FPS games. but games like RTS 30FPS is already good enough. so in the end it depends on the game as well