I know this has been asked several times but I can't find a decent answer when searching.

I'm not exactly running a cutting edge setup: Toshiba 42HL67 720p LCD TV, Onkyo 504 receiver (I think) and Acoustic Research speakers; and I just picked up a new 360 that has HDMI. I've been running everything through Component video/optical audio but I'm thinking I might move to HDMI. What's the best way to do this, HDMI to TV or HDMI to receiver? Can I run HDMI to TV and use the component cables (which has the plug for the optical sound) to run to the receiver, or will it look/sound better just going all HDMI?

Basically you have four options:

1. Component from XBox to TV, optical from Xbox to TV
2. Component from XBox to TV, optical from XBox to receiver
3. HDMI from XBox to TV, optical from XBox to receiver
4. HDMI from Xbox to receiver, HDMI from receiver to TV

Number 4 will obviously only work if your receiver accepts HDMI. Regardless, you won't see/hear much (if any) difference between options 2 and 3. Both of these are preferable to option 1, since your receiver and external speakers will (hopefully!) sound better than your TV's built-in speakers. The Xbox will output 1080p video over component, and 5.1* audio over optical, so you're really not gaining much by using HDMI except less cable clutter (option 4).

*HDMI supports up to 7.1 audio, but AFAIK the Xbox outputs 5.1 max

My receiver accepts HDMI (although I can't take component from the 360 to the receiver and then HDMI from the receiver to the TV...). Will I lose anything by pushing the video through the receiver instead of straight to the TV (even if I am moving from component to HDMI)?

I'm guessing you have the Onkyo 505, as the 504 is an older model power amp. If this is the case, you will lose nothing by sending the signal via HDMI through the receiver to the TV. The 505 supports full HDMI 1.3 passthrough. My setup works exactly like this with an Onkyo 805. Just make sure you have decent HDMI cables (e.g., monoprice.com).

Monoprice is where I go for all my cables. Much thanks for the info!!!