You're already running
- Intel Xeon (5160) (4 cores @ 3.0Ghz right?),
- 16GB RAM,
- ATI 5770.
So there's no hardware upgrade that's going to help. (Well, maybe a 570 for some CUDA clocks but...)
LR is a dog on any machine. You can go for CaptureOne or something to fix that one up. Photoshop itself is also 2 to 3 times faster than LR4.x Bridge is between 8 and 22 times faster - depending on the task. (these are actually tested times BTW).
And you don't need a "more recent mac" for photo editing. I'm very into photography and edit 100's to 1000's of images in a session and the machine I use for that is a MacPro1,1 8x2.67GHz cores, 32GB RAM, and a 8800GT card. It's your tuning... I can /almost/ guarantee that if you were to migrate your current set-up over to the newest MacPro5,1 you would still have lag issues.
My site:
http://tesselator.gpmod.com/Images/_Image_By_Lens/
Or start from here:
http://tesselator.gpmod.com/Images/
Just some of my equipment:
And my Mac with slower specs than yours edits photos at about the same speed as the maxed out MacPro4,1.
Geekbench is the lamest benchmark on the planet. I know it got popular somehow but its "score" is both inaccurate and virtually meaningless. My GB scores for example, can vary from minute to minute or day to day by as much as 600 points. It's OK-ish for looking at the various component tests individually tho. For example I can see that my RAM (or at least the bank GB used) is operating at normal speeds for this machine:
- Memory Section: Stdlib, Write single-threaded scalar, 1497, 3.10 GB/sec
and so on...
But as an indication of general performance GB is almost a complete failure.
.