I think any of the current configs in the Mac Pro and MacBook Pro lines would be more than sufficient. Single image editing isn't too bad on most computers, but video is what will really get you. The upgrade for you from a G5 may be hugely noticeable or only minor. It depends on what version of Photoshop you were/will be running, and how old your G5 is. Make sure you get extra RAM (I'd go 4gb minimum), and a faster hard drive (7200 rpm) doesn't hurt either, especially if Photoshop uses up all your memory and is forced to scratch disk it.
That said, I run a 17" MBP (latest version), with 4gb RAM, but I wouldn't call Lightroom "smooth without pause." I think for now, with Lightroom still working out some performance tweaking, when you have 20,000+ RAW files in your library, things are going to tend to slow down no matter what. Once you move from browsing hundreds of image thumbnails (Library) to actually just working on a single image (Develop), things speed up very nicely; no delay in the edits. Much has been posted in the Adobe forums about Lightroom's lack of speed lately, regardless of particular computer.
Hope that helps.
-J