THe original Xbox version is "ok"
Its the same game (mostly) but it suffers from some graphical ugliness that will not be in the 360 version.
As to why you should get a 360, its 450 bucks, but you wont'have have to respend it every 6 months to keep up with the newly released games on ram or video card upgrades, it will let you play MOST of the original Xbox games AND the newly released 360 games.
The downside though is you will need to pay ~60$ a year to play the online games with people (except MMO's). The games themselves are also ~60 bucks as opposed to 20-50 for the original versions.
If you have an original Xbox, HL2, Doom3 and Far Cry are all good "PC games that need beast-PC's to run) games. HL2 is the worst looking of hte bunch on the original Xbox though. So be forewarned.
I cannot recommend the 360 highly enough though. Some damn fine looking games on it. And the HL2 pack sounds like it will be worth it.
As for the shill-bashers, you guys need to get a life.