Given the OP's present equipment HDMI is pretty much a no brainer.
To utilize the HDMI output of the HD-A2 for that oh so great Dolby Digital TrueHD and DTS-HD 7.1 audio you would be best off getting a receiver that can manage that stuff.
I think the Onkyo TX-SR605 is probably the best value and feature laden receiver for under $500.
I would beg to differ on "no-brainer", depending on one's priorities. Your recommendation is a very short-term solution.
(quote from Wikipedia's entry on Dolby TrueHD),
All HD DVD players offer the option to transcode the TrueHD bitstream into a different legacy format (such as Dolby Digital or DTS), providing a high-quality approximation of the original TrueHD audio over the familiar TOSLINK cable.
Therefore, a similarly-priced but higher-quality receiver might be a better long-term value for the OP, even though they might be last year's model and have fewer "features".
This especially applies if he uses a VCR, CD player, phono, radio, or any other non-HDMI device on a regular basis, or might make a future investment in an HD player with external outputs (HD-XA1 or XA2), which could get new Dolbies and DTSes via software upgrade.
In these cases, the OP clearly would benefit from better audio quality in the long run. HDMI does nothing for audio quality on non-DVD sources (we could extend even this to any non HD-DVD sources, as I am personally not aware of any advanced audio on run-of-the-mill DVD discs).
HDMI in a receiver is clearly a one-trick pony, quality-wise.
In the end, it's the quality of the electronics that make a receiver a long-term value. Bells and whistles are very short-term.
Value is only defined by on one's priorities.