Hello there. Let me say this as gently as possible; you're screwed. I'll explain.
Back in January, I built my very first, home-built computer...a smashing success story. Why did I do it? Because I also have a Dell Dimension L550r, that's why! Same boat as you.
I went round and round and round with Dell tech support via email and phone for a week trying to find out if I could just plug in a faster CPU I.E. the 800 that's currently in your box. Their tech support knew substantially less than I did about my L550r. I couldn't get a straight answer from Dell and I didn't want to risk frying a brand new $300 CPU and my Dell's mobo. It should be noted that back then I knew maybe 20% about computers in general as I know right now. Building your own system from scratch is one the most highly educational and rewarding things you'll ever do.
My Dell has the same intel mobo that yours does, but a different BIOS. As you probably have already found out, Dell BIOS's don't let you set anything about your chip. Not FSB, not multiplier, not voltage, nothing. Also, that mobo has no AGP slot, so upgrading to a "real videocard" isn't even possible.
I just refused to believe that I was stuck with an unupgradable pc. I even posted on this forum and was told the same thing that I said on the top line of this post.
Now, you can get a fast PCI card Voodoo 5000 comes to mind. But 3DFX is dead, so forget about future driver upgrades. Buy at your own risk. You can also upgrade the memory (if you haven't already done so) to 256mb. I did that on my Dell and it made a big difference.
I'd say maybe it's time for you to build your own pc. Build it the exact way you want it, not the way someone else tells you that you want it.! Good luck. Cheers!