If I were you, firstly I would download Team Fortress 2 onto your PC to see how well it runs, TF2 is free on steam so if it doesn't work out then there is not big loss. There are things you can do if TF2 doesn't run very well on your PC, such as changing the texture/model/lighting quality in the video options. If it still doesn't run well you can use this website to improve the performance (I also found a tutorial on how to use the website). Both of these things helped me get a good fps while playing TF2 on my PC. I am sure there are other resources you can use to get your computer to run TF2.
If your computer just refuses to run TF2 then I would have to ask how interested you are in the Half Life 2 and Portal series. Both are great, and in my opinion good enough reasons by themselves to buy The Orange Box. Even without TF2 you still get 4 games in the package (Half Life 2, Half Life 2 Ep 1, Half Life 2 Ep 2 and Portal) and you can get a lot of fun out of them. If you are not interested in Half Life 2 or Portal, or you have already played them and you are only buying The Orange Box for TF2 then I would probably advise against it. Again, I would insist on trying the steam version of TF2 as it is definitely better since it is free, has semi-regular weapon and map updates, and still has a huge number of people playing it. You can still find matches on the 360 TF2 at the right time of day, but who knows how long that will last, and there are no updates.
On the other hand, if you are even slightly interested in the greatness that is Half Life 2 and Portal then I would say buy it. You can get it pretty cheap because it is a few years old. I don't know where you live so I can't tell you the best place to buy, but if you don't mind shopping on the internet you can get it for 28 dollars on Amazon, or cheaper if you don't mind a second hand copy.