Well definitely you'd see an improvement with the newer sub $100 cards. They can handle audio and video decoding better (hardware accelerated). However, most video purists prefer to use CPU rather GPU because it produces a marginally better picture. For me, the difference is entirely rubbish, I live with DVDs and could never justify the higher price of Blu-Ray.
For sub $100 and 100% concerned about quality I'd go buy a GT430 for the 3d support.
The GT 430 is not as bad a HTPC card as the HQV benchmarks might lead one to believe. It is the first sub-$100 component to enable HDMI 1.4a and playback of all 3D formats. In addition, users can also expect 3DTV Play to enable them to play 3D games on a TV when NVIDIA releases that software package in the future. However, the suitability of the GT 430 for gaming purposes is covered elsewhere in this piece. From a HTPC perspective, it enables cutting-edge technology without any sort of price premium.
For a consumer interested in the 3D ecosystem, the GT 430 is definitely a better option than the similarly priced HD 5570. The GT 430 also brings the latest and greatest of VP4 to the table. This includes full MVC decode acceleration enabling hassle free playback of 3D Blu-Rays. The GPU also scored a perfect 100% in our media streamer test suite, and had no issues with bitstreaming HD audio of any kind. Flash acceleration works very well and sites such as YouTube and Hulu benefit handsomely. Silverlight also utilizes GPU acceleration. Netflix is able to take advantage of the same.
See, you don't need much money to buy a card that can decode HD video well, it's just with more powerful cards you have more shader processors which lets you put more filters and whatnot onto your video.
More powerful cards will net you a little gain, but not much. Features tend to be more important that raw shader power. I'll be honest, all of these cards are enough to play Blu-Ray. There's only so much shader power you can need.
It's like getting a hex-core for Office. Sure it might be that little tiny bit faster, but is it more worth it over a dual-core CPU? Nope.