Hey there, currently the 3 optional models for North America and western Europe (France, UK mostly) consisting of the following setup:
G20AJ-US009S = a i7 CPU & Equipped with NVIDIA 760 GTX (Top Tier)
G20AJ-US023S = a i5 CPU & Equipped with NVIDIA 750 GTX (Mid Tier)
G20AJ-US006S = a i3 CPU & Equipped with NVIDIA 745 GTX (Low Tier)
Depending on the budget and what you looking for to use mostly, I would recommend the i5, and upgrade the gpu and cpu later on at the fall of 2015, or perhaps before if you willing to spend extra bucks on the side. Going for top tier model works but if you planning to upgrade, its current price would be more of a waste to do so.
Each of them also have minor differences in terms of HDD and depending where you buy it, Keyboard and mouse will be included (some retailers will remove the illuminated ROG traditional G20 keyboard for a regular ASUS keyboard, so watch out)
These models are from mostly the January cycle of 2015. However, ASUS has now updated them with the 900 Series in certain countries such as Norwegian, eastern Europe. They should also come in North America this summer hopefully.
I Purchased the i5, and planning to throw a 970 in there this summer. the CPU is perfectly fine for me, so I don't see myself upgrade it till Christmas imo. In the end, I may spend a little bit more compared to a i7 model but for way more performance.
FYI upgrading would void your warranty.