If the options are adding another matching sub or new surrounds, I'll take the sub all day. More immediate gratification.
But I would look into a new complete front soundstage. I think your money would be better spent there.
HD-DVD: 55 / Blu-ray: 83
newest: Brotherhood of the Wolf / The Raid 2, Book of Life
AVR: Marantz SR6006
F/C/R: Polk RTi A9's / CSi A6 / Speakercraft AIMWIDE 3 / Sub: SVS PB12-NSD
Samsung PN58C8000 Samsung BD-P2550, Toshiba HD-A30, SA 8240HDC, Xbox360 Elite, PS3, WDTV Live