9800GT Green Edition is slower than a reference 9800GT: http://www.guru3d.com/news/nvidia-geforce-9800-gt-green...
"a GeForce 9800 GT Green Edition, which carries forward the design methodology of the 9600 GT Green Edition: lowering the GPU voltage and slightly reducing the clock speeds. It is expected to carry clock speeds of 550/1375/900 MHz (core/shader/memory) against the reference 9800 GT specifications of 600/1500/900 MHz"
But still its faster than a GT 240
No in terms of raw performance,9800GT is a faster choice
not by a lot as 8800gt-9800gt is faster than 9600gt-gt240 by 10-15 % and green edition 9800 is slower than standard, so don't expect much difference in performance, and considering it is newer architecture ( gt240 ) i think it will be better compared to older 8800 by time when nvidia starts using dx10.1 and dx11 ( when they release fermi ).
The GT 240 is weaker than a 9600GT. The 9800GT green or not is a much better choice. If the latest tech/DX is the goal at the expense of performance then the HD5670 would be the better option but I'd still recommend the 9800GT instead.