nVidia Card Guide
If you want to buy a specific nVidia Card, you should compare it to an AMD Card at the same performance level first.
Pros:
Very Good Driver Support
CUDA Support + PhysX
More Aimed at High-End users
Very Good at 2D Tasks
Extraordinary OpenGL Performance
Cons:
More Power-Hungry
Produces More Noise than its AMD Counterparts
Not Very Good at DirectCompute (eg. HLSL)
1- Numbering Confusion
Novice users are usually very confused about Graphic Card numbering, and sometimes get tricked by the company.
The first number of nVidia Cards (340, 460, 560) signifies its Generation, higher it is, higher the chance that your graphic card supports more complex extensions, like higher DirectX and OpenGL Versions. Note that even if your card's generation is higher, it doesn't mean better performance!
2xx, 3xx = DirectX 10 + OpenGL 3.3
4xx, 5xx = DirectX 11 + OpenGL 4.2 + OpenCL 1.1
The second number (520, 560, 580) signifies the performance rank of the graphic card. Higher it is, better will be the performance in 2D and 3D applications.
nVidia also add a prefix and a suffix to their cards. GT prefix is for entry-level, GTS is for mid-range, and GTX is for mid-range and high-end cards. The Ti suffix is usually for an ameliorated version of the same card.
1-2 = Low Level cards, only suitable for 2D applications and are just a little more performant than Intergrated GPU's. (eg. GT 510, 520)
3-4 = Entry Level cards, suitable for playing games with medium settings at 720p. Not very performant for heavy 3D Modeling. (eg. GT 530, 545)
5-6 = Mid-Range cards, suitable for True HD (1080p) gaming at high settings. (eg. GTS 450, GTX 560)
7-8 = High-End cards, suitable for Multi-Screen (3D Surround) gaming at ultra settings. (eg. GTX 480, GTX 580)
9 = Dual-GPU cards, suitable for Multi-Screen (3D Surround) gaming at ultra settings and Very High AA levels. (eg. GTX 590)
P.S. nVidia releases the most powerful cards first and then does some lower-end cards afterwards. This ends up making lower-end cards that has better features and smaller architecture than the higher cards. (But does not mean that lower-end cards are more powerful!)
The third and last number (550, 555) is a performance rank between cards that have the same second number. Higher is the number, better will be the performance.
2- Power Consumption and PSU choice
nVidia usually recommends a wattage far superior that what is really needed, since most people have a no-name PSU with bad power efficiency and cheap quality.
It is recommended to buy at least from a branded company like FSP or Seasonic.
But if you really want a good quality and lasting PSU, choose some reputable brands like Corsair, Thermaltake or Antec.
Here is a table that tells you the average required power supply for each card.
Please note that nVidia cards with the 2nd number lower or equal to "4" does not require a 6-Pin connector.
Also, a graphic card that is not in the same generation but with the same performance level consumes a very similar amount of power. (eg. GT 240, 440, 545)
GT 510 to 545 -- 250W to 300W good quality PSU Recommended, otherwise, 400W is recommended.
GTX 550 to 555 - 400W good quality PSU Recommended, otherwise 500W is recommended.
GTX 560 to 580 - 500W good quality PSU Recommended.
GTX 590 ------ - 600W good quality PSU Recommended.
3- Special Features
There are special features like 3D Surround, PureVideo, etc. Keep an eye on those features if you need them. Features are usually listed by the seller.