Heya FlyBill,
Welcome to SimV! Just for comparison I am currently running a custom-built PC (I built it myself) with an Intel 2.4 GHz CPU, 1 Gig DDR RAM, GeForce FX 5200 128 MB, and Windows XP SP2. So my PC is roughly equivalent to yours (I don't know about the graphics card).
If you wish to know everything about your PC (but were afraid to ask) go to Start, then click Run. In the entry field type
dxdiag (uppercase or lowercase -- it doesn't matter). Then click OK. You may wish to click the Save All Information button at the bottom of the Dxdiag window. Just save it on your C:\ drive. This will place a new text file on your hard drive called dxdiag.txt.
You can open the Dxdiag.txt using Windows Notepad and read all of your PC specifications including graphics card type (under the Display Devices section), the drivers you are currently using, your DirectX version, hard drive capacities and amount of space used, etc. You can refer to this Dxdiag text file any time you have an issue and someone on this board wants to know your PC specs.
In my opinion there is no comparison between FS 2004 and FS 2000. The latest version of FS is infinitely better! I should know since I've owned and used the Flight Simulator (and all of its various incarnations) since the very first version was introduced about 20 years ago.
Since our PC's are similar you will most likely get very good framerates (without any Add-ons). I have all sorts of Add-on Aircraft, Scenery, Sounds, etc. and I get 30+ FPS in the city and 45+ FPS in the country (With all of the Graphics Options sliders maxxed out).
You did mention in your original post that you have the SP2 upgrade for Windows XP. This is a very good thing. When I upgraded XP to SP2 I didn't notice any real increase in FPS, however, I did notice an overall improvement in the way the scenery textures looked.
One recommendation I would make is to download and install DirectX 9.0c (if you don't already have it).
If you don't have it then you can download it here:
http://www.microsoft.com/downloads/details.aspx?FamilyID=0a9b6820-bfbb-4799-9908-d418cdeac197&DisplayLang=enOn my PC I noticed that it did increase my framerates by about 10%. Not much -- however, with as much stuff as I have on my FS 2004, every little bit helps!