Need some advice on an older system

As a spare computer and a secondary computer, we have an eMachines T1090
with a 900MHz Intel (Celeron). Right now, it has the onboard Intel 82810
graphics controller. We can play Quake III Arena with that, (although we do
get a message when you open Quake) but the only way to get it run even
semi-smoothly is to set the screen resolution to something less than 640x480
and to set all settings at their lowest. The eMachines only has PCI slots -
no AGP or PCI-E. First of all, what's more the slow-down - the onboard
video, the CPU, or both? The eMachines has 384MB of SDRAM and only a small
amount of programs installed. If my goal was to get Quake III to work
smoothly at least at a 640x480 screen resolution on medium settings, would
it be possible with this system with just an upgraded PCI video card or is
it worth the time and effort at all? Secondly, any ideas if the Bestec PSU
can handle it? Quake III is the only game I'm concerned about - no plans to
run Doom 3, obviously. Thanks

Specs (nothing special):
8x4x32 CD-RW drive
Floppy drive
Windows XP Home
120GB Seagate 8MB cache 7200RPM HD
384MB of SDRAM
Intel 82810 onboard graphics