Upgrading old computer: what should I fix?

I'm trying to make my computer optimal for a media center: movies, music, etc. Basically, I just want it to start-up faster, and run media files smooth.

Here are the stats:

Dell Dimension 8400

Windows XP Professional

Processor: "x86 Family 15 Model 4 Stepping 1 GenuineIntel ~3192 Mhz (3.2 Ghz processor, single-core)

Total Physical Memory: 1024 MB

Available physical memory 511.48 MB

Total virtual memory 2.00 GB

Available virtual memory 1.96

Page file space: 2.40 GB

Sound card: Creative Audigy Audio processor (WDM)

GPX Card: ASUS EAH3450 Series (512 MB RAM)

Hard drive: 144.31 GBs

Free Space: 54.46 GBs

I've already upgraded the sound card and the graphics card. I'm thinking about upping my RAM to 2 Gbs of 1024MB-667MHz DDR2-667 PC2-5300, 240p DIMM, 1.8v (by Elixir). Any other suggestions?