Is this is a good card for my 2008 Mac Pro?

Yes, that's a good card. A compromise of decent performance and price. I have the GTX 650 and it works well for me. The Ti has better performance than mine so you will be happy with it.

If he says yes, great and there us GTX support out of the box? I have seen others with issues though on MACRUMORS forum with 660 and TI and you dobwant 8.3, maybe because a 650M us used in other Macs it works well but, boot screen ?

No. Download http://www.nvidia.com/object/macosx-cuda-5.0.45-driver.html instead.

Frankly, I can't understand what he is saying. It makes no sense to me.

The boot screen issue is pretty well known. I have an original Mac Pro in which I put a Radeon HD 5770 non-Apple card. There was no video until the Desktop was deployed. A bit disconcerting. I happened to have a monitor with both DVI and VGA inputs so I ran a VGA cable and DVI cable to the card. When the computer booted the gray screen displayed but in VGA mode. At least that made it possible to use startup commands. You could give that a try if you have a monitor with a VGA port.

Yes, if there's no boot screen to see you cannot use OPTION booting.

Well, I think you know what your options are as well as what will be missing when you use one of these cards in a Mac Pro. They do have VGA ports, so if your monitor supports it then you can basically drive both the VGA and DVI sides with the same card.

Oh, one thing I did forget to mention is be sure to check the type of connector on the card and what you have on your monitor. This might dictate a cable change. I recently bought a new monitor that used a dual-link DVI port so I had to buy a new cable to use. Took me a while to figure out why the cable I had already wouldn't work. Silly me.

There seemed to me to be so many variables with various models, whether you got video, what ports will function as sometimes only one DVI, that there is no simple answer on some. With the GTX 670 it seems easier, or just check and go with one from Macvidcards. Not all Nvidia cards have same size firmware and features and of course is there a device ID for it to be recognized and load extensions.

So even if it has two DVI you may not get output from both.

There is a lot of reading and information in a couple of threads on MacRumors Mac Pro forum.

I got the EVGA NVIDIA GTX 650 Ti Boost. The reviews said that it was kinda noisy, but it's not, at least not compared to the whining lawnmower Radeon HD 2600 I had. People have said that using a non-Mac GPU will not allow you to use bootup keys. Despite there being no bootup screen, I CAN use bootup keys! I got it into Target Disk Mode normally by holding "t" at bootup, for example. Option might even work, but you wouldn't be able to see what you're doing.

The only real problem I've noticed so far is that it doesn't seem to want to sleep when I tell the computer to sleep through the  menu. I'm going to look into it further.

Does anyone know if there are NVIDIA drivers or control panels we could install for Mac? I know that in Windows, you have to use those to change any display settings, but they also give you other options and typically the GPU temperature. The Displays prefpane is supposed to do the job of a GPU control panel, but it lacks card-specific features.

I don't know where you read that you can't use startup keys, only that you don't get an early boot screen.

I have read of sleep issue on MacRumors forum. You might ask and see, some send theirs to MacVidCards, some did their own.