the number of stream processors and the clock of the vidio card. but you can't compare the number of these parameters between generation so you cant compare a hd 5xxx whit a hd6xxx because the architecture is different.
nor can you compaire these numbers between nvida and amd because it is totally different.

(nvidia is high clock lower amount of stream processors and more complex proccesors while amd is the other way around)
so look for benchmarks.
the higher the number the better the card and ti for nvidia means that it is a better card.
for what i have heard is that nvidia is better for folding and gpgpu. but amd is now more power efficient.

It's probably best to search for benchmarks to compare video cards to the apps you are trying to run. Make sure that the card has the newest video card memory and not some old crap like DDR2. Also get as much memory as you can because that will help with your 3D modeling. Stream Processing Units matter and is something to consider when picking video cards. Stream Processing Units for the most part can be like going from a V4 car engine to a V12, depending on what you are running such as video games that use DX11. Try searching Tomshardware. http://www.tomshardware.com/charts/graphics-cards,1.html

Thanks for the help

Okay so lets say your trying to decide between EVGA's GTX 550 TI and its GTX 560 TI

GTX 550 TI Specs
Model: 02G-P3-1559-KR

PCI Express 2.0 x16

Chipset Manufacturer NVIDIA

GPU GeForce GTX 550 Ti (Fermi)

Core Clock 900MHz

Shader Clock 1800MHz

Stream Processors 192 Processor Cores

Effective Memory Clock 4008MHz

Memory Size 2GB

Memory Interface 192-bit

Memory Type GDDR5

DirectX DirectX 11

OpenGL OpenGL 4.1

Ports HDMI 1 x mini HDMI

DVI 2 x DVI

RAMDAC 400 MHz

Max Resolution 2560 x 1600

3D VISION Game Ready Yes

SLI Support SLI Ready

Cooler With Fan

System Requirements 400 Watt or greater power supply recommended

Power Connector 6 Pin
Dual-Link DVI Supported Yes
HDCP Ready Yes
Card Dimensions 8.25" x 4.38"

========================================================

GTX 560 TI Specs
Model: 01G-P3-1461-KR

PCI Express 2.0 x16

Chipset Manufacturer NVIDIA

GPU GeForce GTX 560 (Fermi)

Core Clock 850MHz

Shader Clock 1700MHz

Stream Processors 336 Processor Cores

Effective Memory Clock 4104MHz

Memory Size 1GB

Memory Interface 256-bit

Memory Type GDDR5

DirectX DirectX 11

OpenGL OpenGL 4.1

Ports HDMI 1 x mini HDMI

DVI 2 x DVI

RAMDAC 400 MHz

Max Resolution 2560 x 1600

3D VISION Game Ready Yes

SLI Support SLI Ready

Cooler With Fan

System Requirements 450 Watt or greater power supply (Minimum recommended power supply with +12 Volt current rating of 24 Amps.)

Power Connector 2 x 6 Pin
Dual-Link DVI Supported Yes
HDCP Ready Yes
Card Dimensions 8.25" x 4.38"

I see that the 560 has better specs in the areas you talked about but the 550 had double the memory size, does that even things out, does it even matter that it has double the memory size when it has 144 less Stream Processors?

Is it worth the $30 price diff? 560 being the one that cost more...

Mind you, the 560 is Super Over Clocked, I'm worried if it'd runs a bit hot in my computer, I have 6 fans no smaller then 120m in my case but it does not allow very good wire management so that may not matter.

i think you can better first look at your budget. like i have xxx dollars to spend witch cars cost les or equal to that xxx than search for benchmarks and temperatures (the lower the better) and so on of the cards.
this might help if you have shearched for the card that fitted in your budget.

http://www.xbitlabs.com/articles/graphics/
http://www.bit-tech.net/hardware/graphics/

if your cards isn't in there list than search on the internet

Heh, well there is a little more to it, I'm restricted to buying from a store, cause my money I have is in the card for that store, my budget is $200, the store is well, Best-Buy heh.

Looking at the specs... The 560 Ti will be at least 22% faster overall than the 550 Ti, regardless of the amount of memory. Double the Shaders and 256bit over 192bit will make a larger difference in games and apps. Most of the 560 Ti's will have a cooling design that sucks air in the center and pumps it out the rear of the case if heat is an issue to consider.

If you can avoid it I would recommend buying a video card from somewhere other than Best-Buy. The main reason is that they only sell the generic reference design from NVIDIA. They also are overpriced there. But to answer your question I would highly recommend the 560 the card works amazing and can handle pretty much everything. Too bad its on a card for there. See if you can haggle with them a bit on the price. I usually get all my hardware from the Egg, they have the best prices and shipping is usually free.