selling my late 09 Imac and guy asks is this a significant upgrade over my G5?

I'm in the position of selling it again. The buyer wants to know if it is a significant upgrade to his current g5 imac?

His:
Hardware Overview:
Model Name: iMac G5
Model Identifier: PowerMac12,1
Processor Name: PowerPC G5 (3.1)
Processor Speed: 2.1 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 512 MB
Bus Speed: 700 MHz
Boot ROM Version: 5.2.6f1

Mine:
"21.5-inch LED-backlit glossy widescreen display
4GB memory "mine is 8"
500GB hard drive
8x SuperDrive (DVDÃÆ'‚±R DL/DVDÃÆ'‚±RW/CD-RW)
NVIDIA GeForce 9400M graphics with 256MB memory
Built-in iSight camera

Features:
/ 3.06GHz Intel Dual Core Processor
/ 500GB Hard Drive, DVD SuperDrive, 8GB! 1066MHz DDR3 SDRAM, plus room for much more!
/ 21.5" LED-backlit display with a widescreen 16:9 aspect ratio, 1920x1080 HD resolution, NVIDIA GeForce 9400M
/ Mac OS X v10.6 Snow Leopard, includes new wireless Magic Mouse

I want to be honest and feel right about the deal and I know squat about g5's

btw I am asking $650, is that fair, too high? about right?

 

that benchmark site is helpful, those others are garbage though, that's what they will buy them for sight un-seen, probably at a 30% cost factor, meaning they pay about $300 and resell for about $900.

At least in my market those prices are about 1/3 what I see daily on craigslst

 

this what I told him:

I think my Imac, while not the latest and greatest is still very modern and is very much a significant upgrade over yours.
I don't want to pretend like I know everything about hardware but on first glance mine has more cores, way more memory, faster speed etc...

But it really depends on what your doing, if you will feel a significant increase in speed or processing etc.. I bet mine boots up a lot faster, and runs programs a lot faster and my operating system is faster etc.. but if all your doing is surfing the Internet or something there are other factors.