Use the original ATI-Driver (Catalyst 4.4) for your OS located on this Page:

http://www.ati.com/support/driver.html

 

Dimension 8300
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P4 3.0 GhZ - HT
1024 DDR (2x512MB)
Ati Radeon 9800 (non Pro)
Soundblaster live digital
120 GB SATA (Seagate-7200U-8mb)
16X Dvd Rom (Samsung 616T)
4X Dvd +R/+RW Writer (NEC-1100A)
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FRITZ!Card DSL SL (AVM)
20" HP A4033A

Thanks.  My only concern is that Dell will hassle me should (no, when) I need support.  They did this when I had the ATI drivers from ATI installed on my laptop.