If you have 512 MB and aren’t running a ton of programs in the background, the available memory size will not be the bottleneck when loading maps. 512 MB is plenty, so increasing to 1 GB will not yield any significant results (as you’ve discovered). By the way, setting com_hunkmegs to values over 192 is not recommended (and, again, will not improve things anyway).
If you experience long loading times with your setup, then you’ll have to look for the cause elsewhere (i.e. hard disk performance, disk fragmentation, IDE drivers, driver settings, UDMA mode selected, etcetera).