Advice upgrading my Dimension 8200

Was wanting some advice as for upgrading my Dimension 8200, Pent 4, 1.8GHz. It came with 256MB PC800 RDRAM and I have upgraded that to 1.5GB. It had a SB Live! 1024 Digital Sound card and a 64MB DDR NVIDA GeForce3 Ti 200 Graphics Card with DVI. I put in ATI RADEON 9250 256MB DDR Memory 128-BIT. I also had a CD-RW and DVD-R and have put in a AOpen DVD+R/RW 16X, 8.5GB.
I would like some advice as to what I might do to improve it for my needs. I use it for gaming, down loading and burning music, movies and pictures. I want to tie eveything together, my computer, TV and reciever if I need one? into a surround sound system with a real nice bass. Any idea where I might start? Thank you!