You are probably correct about that. Two HD streams might not be possible even with a wireless N router.
G can handle 1 stream (you really only get about 20-25mbps out of '54'g), N should be able to easily handle 2, with a conservative estimate of 50% available bandwidth on half-duplex mode (75mbps).
And before anyone says 'no, G won't work', I have an HDHR. and I've streamed every channel without issue. Good quality signal and hardware are required, but that shouldn't be an issue if you're buying new.
Remember, you're not streaming the uncompressed video data (like an HDMI cable), you're streaming a max 19.4mbps MPEG2 file.