DVD quality is 720 (width in pixels) x 480 (height in pixels) 30 frames per second, so the 640 x 480 30fps is near dvd quality, and is perfectly good for watching on a tv.
The 1024 x 768 is the standard resolution of a non-widescreen computer monitor, and is more than what you need for tv playback, so in effect it's a waste of memory to use it, as you probably won't really notice the difference. In fact it might seem a bit worse due to the lower frame rate as motion will not look as fluid.
320 x 240 is fine for internet usage, but you're better off to shoot in a higher resolution and recode it as necessary. You can reduce good quality video, but you can't improve low quality.
Video is a memory hog. On my N95 phone 640 x 480 30fps uses over 20mB per minute of video. Buy a few medium size memory cards and you'll be fine - I always prefer to have a few mid sized cards rather than have all my eggs in one large card.