which modelling and rendering software shall i get

I am an architect who wants to develop my portfolio with some nice renderings. I am not sure which software to get. I use a PC and AutoCAD. I used 3d studio many years ago at uni but its so expensive. I know there are lots of different options but my problem is that having not used any of them I don't know where to start and in don't have alot of time to investigate each and every one of them. Google sketch up is quite cheap but as far as i know its rendering is limited. I want to produce some photographic 'realistic' resolution Images. Would appreciate some firm guidance. Thanks.