will this be able to watch and record HDTV with eyetv?
Well, there are a couple of issues here. First, that Hauppauge, while identical to El Gato's Hybrid, and supported by EyeTV software, doesn't actually ship with EyeTV software--so you'll need to buy a license. Probably should just get the Hybrid. Second, no, your Mac won't be able to play back high def, it'll play it back at 1/4 res with EyeTV. Third, RAM won't help this issue, it's a processor issue, but any Mac user running Tiger should have a least 1GB in their system, so pop another 512 in there anyway (especially since another 512 stick of that RAM is like $30.)
Antenna stuff is already handled very well in another AVS discussion forum called "HDTV Reception Hardware"--there's nothing about an antenna that is a Mac specific issue--and yes, many folks use very inexpensive Radio Shack antennas to pull in their OTA. I have a Silver Sensor, but what'll work best for you is very locally dependent--so you may want to drop in on your local HDTV discussion thread to see what others are using near you. All depends where you live, where your towers are, what's in the way, etc. You won't know what any antenna will pick up until you connect it, though antennaweb.org can give you an idea of what to expect.
also, i wanna hook everything up to my denon avr-1403 receiver. what is the best and cheapest way to do so? what cables do i need?
Your M-audio card has digital coax out, so just go coax in to the Denon.
(And, by the way, you had some mad crossover skills back in the day.)