I will make a suggestion:
If Portal runs well on your PC, please do yourself a huge favor and buy The Orange Box. You will get Portal. You will get the superb Half-Life 2 series. You will get the fantastic Team Fortress 2. And all for $30.
However, I'd hold back on buying it right away -- the Engineer update should be coming out soon, probably by the end of May, so Team Fortress 2 and The Orange Box should be on sale. That would be the time to buy.