TuneUp Utilities actually accomplishes a couple different things when performance tuning your machine. Most notably, a registry sweep. With TUU, you can not only get rid of unnecessary registry entries, but also defrag the registry. This adds a little performance boost in the machine, but as stated above, a SSD will make the change very noticeable. As for which one to buy.... I can't be too sure but don't go for capacity on SSD's as you really only need the OS/Program Files installed on it. You can then use your existing drive as a data only drive (music, docs, downloads, etc...)
Here's a comfortable 90GB SSD for about $184 before tax+shipping: Newegg.com - OCZ Vertex 2 OCZSSD3-2VTX90G 3.5" 90GB SATA II MLC Internal Solid State Drive (SSD)
Here's a comfortable 90GB SSD for about $184 before tax+shipping: Newegg.com - OCZ Vertex 2 OCZSSD3-2VTX90G 3.5" 90GB SATA II MLC Internal Solid State Drive (SSD)