Should I buy a new better graphics card and use it with my old one or just use the new one?

if use nvidia gpu supports sli*, newer nvidia graphics card supports sli may supplement nicely... if use ati (that supports crossfire*), ati cart supports crossfire might work. cannot mix ati , nvidia cards , expect them each other. stick 1 brand. (explanations below.)

==to add another==
first thing check, whether or not have open expansion slot. next check whether or not pc can support another. (some pcs have enough slots , don't have powerful enough power supply. motherboards don't behave crossfire or sli)
if ok, next thing pick graphics card. recommendations below.
next install , configure.

==to upgrade==
remove old one. check whether or not have open expansion slot. next check whether or not pc can support another. (some pcs have enough slots , don't have powerful enough power supply.)
if ok, next thing pick graphics card. recommendations below.
install , configure.

==explanations , recommendations==

*sli , crossfire 2 dual-graphics card systems offered nvidia , ati. these systems distribute work load across both cards, improving performance , reducing strain on hardware. applications and/or games (especially betas , older ones) don't support sli or crossfire.
[grfx card a] + [grfx card b] = better (most of time)
pros: -more power -less hardware strain -sometimes cheaper
cons: -eats more power -puts out more heat

-recommended cards-
please note older cards bunched lower end cards because of similar performance.
nvidia: (overview)
-high end: gt 670+ gtx 670+, gt 580+, gtx 570+ (expensive, best ones replace with.)
-mid class: gt 550-580, gtx 550-570,
-lower: gt 200-390, gtx 200-390, gt 400+, gtx 400+, gt 500-560, gtx 500-540, gt 600-640, gtx 600-640 (anything older 400 series isn't worth buying, don't have enough power useful long. lower class designed basic tasks web browsing, not 3d games. cheap)

ati: (overview)
-high: higher radeon 6000 , 7000 cards , higest 5000 series if wants cheap... (again, best replace expensive)
-mid: mid-high radeon 5000, 6000, , 7000 series high 4000 cheap... (mid-range)
-low: 3000, 4000 series, lower 5000, 6000, 7000 series (eh...)

-comparison-
nvidia graphics cards more expensive ati counterparts, run cooler, use less power, , last little longer on average. ati cards use more power, run hotter , cheaper. performance similar gt , gtx 590 , 690 have double power due dual core layout. twice expensive...

---recommended me :)
-gtx 560 (usually cheap enough, power, sli, proven) ($ 100-500)
-gtx 660 or 670 (ok price, power, sli, )
-gtx 570 or 580 (good replacement cards, excellent power, not expensive, sli) ($ 300-600)
-radeon 6850 (similar gtx 560, crossfire)
-radeon hd 7850 (similar gtx 670, crossfire)

luck!

i read somewhere using 2 different graphics card isn't @ all. should use 1 better 1 according website. true? if not can recommend me?

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