You do have to remember though, that not all 1GB GPU's will be as good as yours. I used to have a Radeon HD 4350, and it was a 1GB card, however it only had GDDR3 ram, which is significantly slower than the GDDR5 found in most GPU's today.
A lot of last gen or older hardware will have the basic quantities of RAM e.g. 1GB but that doesn't mean the ram is fast enough to run Reloaded well. If it were lowered to 512mb minimum, you'd probably end up with lots of people wondering why it wouldn't run on their 2004 gpus, which may have 512mb ram, but are only actually GDDR2/3 RAM too slow to run the software.
A lot of potential buyers of the software are unlikely to know what type and speed of RAM their GPU has, and TGC can't really expect them to research all that - so 1GB seems to be a good lower limit.
Thats my thoughts, anyway