Eric, I highly doubt you will see a difference. The only time I could ever think of anyone being able to notice difference in performance is if you were running one of the next-gen games (Doom III for example) with AA and AF on. Otherwise, this card should work fine for you, plus you'll save...
I've heard TerraTec makes good cards, so you shouldn't have any problems with that choice. I've never heard of the Audigy 2 lagging on playback of movies and MP3's, but I could be mistaken.
I would actually recommend the Seagate's 8MB Cache drive over Western Digital's 8MB Cache drives. Seagates have always had a great reputation for being quiet and dependable drives. I think you definetely have chosen the better drive in this case. You should be fine on the thermal paste unless...