Just for reference, I'm running Orbiter on a 900Mhz Athlon, 768Mb Ram (SDR), and a 64Mb Nvidea G-force (G-force 2 I think), on XP, and I'm getting slightly better framerates than you are reporting. I'd say that your main problem isn't just hardware. Have you tried VistaBoost (available at DBeachy's webpage - where you get the XR series vessels)?
Is the graphics adapter on the motherboard, or in a slot? If it's on the board, it's probably using "shared" menory rather than dedicated video memory. There MAY be a jumper you can move to enable a dedicated graphics card. If it's a seperate card you may be SOL - Dell has been known to use slightly modified connectors for everything from graphics cards to power supplies for the purpose of vendor lock-in.
If you do go with a new PC, I recommend building it yourself, or having it built for you. The advantages are that you can cherry-pick the hardware and you'll have the ability to upgrade components later - unlike many Dell PCs. Building your own isn't all that difficult - just take your time, don't force anything, and use a grounding strap on your wrist. Also, I recommend "Arctic Silver" brand paste for the heatsink. There are many sites on the web that can give you the techniques for assembly. Just do the research and ensure you are getting parts that fit together (ie, the right socket type for the CPU, the right memory, etc.
A "build your own" approach will get you a top notch bleeding edge computer for far less than buying a pre-built PC with similar specs, and you'll have a box that will be usable for far longer than a mass market desktop.