2BG of RAM will make you run, load and verify client data faster because there is more memory, the more RAM you have the more textures the game can load into the memory.
The reson you lag at 1GB is because the game connot load all the textures into memory as there is not enough of it so it has to load them from the harddrive, as a hard drive has slower tansfer speed then RAM this will make you lag.
Solution upgrade to 2GB and turn of windows page file, you will notice a hug differnce.
I upgraded from 1GB - 1.5GB - 2BG it used to take me 1min 30secs to load a on medium settings at 1GB, then about 1min on 1.5GB on medium. Then i finally got the 2GB and i load in about 36secs with all high settings, but i have 2 rapter hard drives in RAID0 aswell so thats probably why.
2GB will make you game run alot smoother, load alot faster and look alot better, its well worth the upgrade as it will boost windows performance, application performance and performance in other games aswell.
Hope you found that useful