Would the Core i3-2105 give noticeably better on-board graphics than the Core i3-530 ?
Yes, they're better than Westmere, but Sandy Bridge's integrated graphics are still significantly inferior to your Radeon HD6450 graphics card. The best Llano integrated graphics are about equal to the Radeon HD6450 graphics card. AMD's "Dual Graphics" allows the APU's graphics processor to crossfire with the Radeon HD6450 for slightly improved gaming performance.
You can see some Sandy Bridge graphics benchmarks vs. the best Westmere processor here:http://techreport.com/articles.x/20188/17
In this review you can see Llano's graphics capabilities compared to Sandy Bridge's:http://techreport.com/articles.x/21208/15
Here it is again with faster memory:http://techreport.com/articles.x/21255/6
Your old Core i3-530 processor and micro-ATX H55 motherboard could serve as the basis for your HTPC (particularly when paired with your Radeon HD6450 graphics card). You could then get a new Sandy Bridge processor and LGA1155 motherboard for whatever more demanding applications you might currently run on the Core i3-530.