You could start by cleaning things up on a software level. Disable unwanted startup programs in MSCONFIG. Run CCleaner and get rid of all the built up rubbish, defragment your drive etc.
Once you've got your OS sorted out, you can determine whether you actually need an upgrade.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.