I have a memory manager (for manually freeing up old apps) on my home page displaying how much free ram is available and on average it shows about 350MB free. That tells me that the quite a large portion of the 1GB ram is indeed being used and also that the 512MB would most definitely need to swap more often. How that translates to performance however is another question as depending on the apps used, swapping might not be noticeable at all. Only having the two side by side with duplicate setups and performing the exact same actions would be definitive, but hazarding a guess I would say the 1GB should outperform the 512MB in most cases where many apps are opened. Again, that might or might not be a negligible difference..
Archos G9 101 Turbo 1.5GHz/8GB
32GB mSD, fw v4.0.7, Android 4.0.3, kernel 3.0.8+-omap4460
HTC Touch One X NL
fw 2.17.401.2 CL89015, HTC Sense 4.1, Baseband 2.1204.135.20
Android 4.0.4, kernel 18.104.22.168-g090b442e