This is how all your posts should be, especially if you are starting a new thread, now there is enough information here to help you
"was just wondering, comparing the 250 to a PS3, which will win in games that come out for both platforms? - Supposingly useing a Quad Core CPU."
It will depend on which quad core in particular in regards to the model and brand. E.g. a Q8200 would perform less than a Q9650 due to it being clocked lower and it having less L2 Cache. Presuming you have a capable quad core the GTS 250 could at the moment run most games today with ease .e.g. high frame rate/high detail with moderate AA+AF. I can not accurately predict how long the GT250 would be suffice for but it should be capable of lasting at least another year considering that there are very few ground breaking upcoming games apart from Alan Wake.
You have to also remember that the expectations for PC and console are different, just because you run a game on a console the conditions in comparison to PC is not the same, on the console you are running at a stupidly lower resolution of 720p (1280x720p) which is sometimes up-converted to 1080i (1440×1080) whereas the average enthusiasts plays at resolution from 16850x1050.
Ask yourself is it fair to compare a PS3 game running @ 1280x720p opposed to the same PC gamming running at 16850x1050 or higher? - obviously if you ran your PC games @ 1280x720 on a midrange card it almost last just as long as a PS3.
Also you have to remember that the level of detail is not the same, PC games even bad console ports have much crisper textures, more CPU players on screen and better AI, to truly compare a PC version of a game to a PS3 version of a game you'd have to turn the detail down to medium settings, and hence the GTS 250 should have about the same longevity as a console.
"A Core 2 Quad CPU, GTS250, 4GB DDR2 RAM"
The specification looks good, but generally speaking a Core 2Quad will cost more than an Athon II or Phenom II build. There is no point investing in a Core 2 Quad for new builds due to socket 775 reaching its shelf life.
"to clear things up, lets take.. GTAIV for example (yes I know, bad port...) Which will run better the GTS250 PC - 360 or PS3"
If you go to "Gametrailers.com" there is videos of people running the PC, 360 and PS3 version of GTA IV side by side and you can tell which one runs better, just ask if you want me to find the video for you.
But to answer that question GTAIV is one of the most intensive games out ATM, not because its graphically fantastic but because the port is bad! Frame rate wise the PC will have a higher frame rate, but it really depends on the level of detail as I said earlier, I would only guess that the PC version of GTA IV on medium would look similar to the 360 and PS3s version, but without knowing which resolution you intend to play at or the level of detail you expect it is impossible to assess your question. Remember the GTS 250 is only a "midrange" card, it is not high end by any stretch so you can not expect to max out the detail of everyday it crosses and medium detail on bad ports and upcoming games should be expected.
Also consider the ATI 5750 as it performs the same as the GTS250, and the ATI 5770 which performs between the 4870 and 4890, both the 5750 and 5770 are much newer and supports DX11 too