No. There is no technical reason you need to go with a 7.1 channel system if 5.1 meets your needs.
This is going to be more confusing but if your TV has HDMI I would connect directly from the HD DVD player to the TV for video and run the 5.1 analog to the 990 for audio.
ANOTHER EDIT: I went back and looked at an earlier post of yours and see you have a 7.1 speaker arrangement so you have what is currently available regarding 7.1. Again I am not aware of any device that outputs in 7.1 that is for the future.
Just a note, since you are dealing with so much that is new to you have you considered the consulting/installation services of a professional installer in your area?
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Outlaw 990/7700
SVS 20-39 PC +
SVS MTS-01 towers, MCS-01 center, MBS-01 surrounds
Behringer A500
Samsung PN58A650, DirecTV HR-20 700
Sony CDP-545, Phillips CDR 765
Oppo BDP-93, Panasonic DMR E515
Technics SL-DL5
Squeezebox 3
Remote Harmony One