So I played the first couple of levels in Portal 2 and want to get a new gfx card. It plays well at 800x600, but with mild settings and stutters a little just when the graphics start to get all pretty. I was also doing a project in After Effects and the refreshes started to get way too slow for to my liking, so I'm putting both those together as an excuse to justify getting a new card.

Right now, I have a GeForce GTX 8600 and want to make a leap up from there.

I'll be going to Best Buy to pick up whatever I end up getting, but I'm confused at all the millions of different number schemes out there (GTX, GTS, 8000 series, 400 series (which is better than the 8000)), and how significant if at all the increases in speed are. So I'm looking at the GTX 450 or a GTX 550 Ti, I don't know if there's a relevant difference. I also have the option to get a GeForce GT 240 for about half the price, but it sounds like a crappier card because the number is so much lower.

Any good advice?

http://en.wikipedia.org/wiki/GeForce#Generations

I'm assuming you want to stay with NVIDIA? In any case a list cards from highest performing to lowest with random prices found on newegg.
Mid Range
------------------------
GTX 560 - 240ish
ATI 6870 - 210
GTX 460 - 185 1GB or ATI 6850 - 175
GTX 460 - 150 768MB

Lower Mid Range
-------------------------
GTX 550 - 150
ATI 5770 - 110
GTS 450 - 115

I wouldn't get anything lower then what I listed for games, and anything faster will probably need you to upgrade your CPU to get the most out of it. The 450 and 550 are the equivelant of the 8600 you have for their respective generations with the 550 being much faster, but still much slower then the 460.

If I were you I'd get a cheap 460 since it's much faster then the 550 and about the same price.

Can you also post your CPU and PSU specs? It'll help determine what would work best.

This ranking may be of help for comparing different models.

5xx is the newer architecture, basically it's a tweaked version of the 4xx hardware. Hence the higher number. However the 550 is a lower end mid range card, and the 460 a mid range, and an older generation mid range can be faster then a lower end new generation card. It's basically all marketing.

The 5xx will use less power to get the same performance as the 4xx. Looking at your specs, I think you can get away with a 460 but it might start straining your PSU a little. The 550 and 460 768mb use almost the same amount of power, the 5770 uses a lot less. Some numbers here:
http://www.anandtech.com/show/4221/nvidias-gtx-550-ti-coming-up-short-at-150/16

I'd recommend using Anandtech's charts for comparing GPU performance (they have customizable graphs) as they are one of the more technically inclined tech sites around.

From the things I heard, ATis current generation beats Nvidia in the ~ <USD200 price range.

Append: GFlops aren't the thing to compare: you need real game test results.. FPS.

Append1:
Enjoy!

In my opinion passive cooling for a GPU should be avoided: these days OCing this stuff is almost vital, and you want a fan for that purpose..

... lolwut?

If it's one component in your computer that should be passively cooled, it's the graphics card. The fans they use for those are usually ridiculously loud.

You can add at 10-30% performance for free, literally. And while you're buying a gaming card and paying for it, that is typically worthy. It has nothing to do with OCing a CPU, & it's a lot simpler task.

Loud or silent.. That's up to the manufacturer. The fans can also be tweaked, and typically you can have them below the noise level an ear can hear while the GPU is idle. While it's running, oh well.. Hear the power

Append:
@decepto No idea what about OGL, but DX11 is supported by everything out since 2010(including 2010), I could be wrong but that's really not likely.

My PS3 runs it fine.

I haven't played single player yet, but split screen co-op was good for the 5 hours I played at a friends house, then we got annoyed at a level we were having trouble with, and I had to get home to bed for work anyway.

Haters gotta hate.

Except it's annoying using a controller and sitting so far away for a first person game. The controller thing is obvious and I notice it in every game. Playing a sequel to Portal on PS3 (when I played the original on PC) made me realize that I also felt more immersed on PC; perhaps in part to being nearer to the screen. Of course, I also find the ability to see portal outlines through walls rather immersion-breaking. I wish there was a configuration option to disable that. It is so far a pretty awesome game; though I think I somewhat agree with ML. It is so far a little bit underwhelming considering my excitement for it. Hopefully things get better. I guess I expected much more story instead of mostly test champers (so far). Not that I don't like test chambers, but it should have been relatively easy to create just more of those... I hope there's more.

Ah yes, Portal. The game where only the last level was interesting, until the Shodan rip-off appeared. sigh

I think whatever my next video card ends up being it will be passively cooled. Less noise is always a good thing, and I don't bother OCing any of my cards.