Try using these settings.
Your problem occurs because of your audio plugin.
Use the Eternal v1.4.1 on my sig below.
Using Eternal 1.4.1, set to SPUasync, Smooth with a buffer of 32.
It is normal to run it from the cd-rom drive.
Try using the Pete's OGL2 Plugin. They do well with the Radeon 9000.
Plugin: Pete's OpenGL2 Driver 2.7
Author: Pete Bernert
Card vendor: ATI Technologies Inc.
GFX card: RADEON 9000 DDR x86/MMX/3DNow!/SSE
- 1024x768 Fullscreen - NO desktop changing
- Internal X resolution: 1
- Internal Y resolution: 2
- Keep psx aspect ratio: off
- No render-to-texture: off
- Filtering: 6
- Hi-Res textures: 2
- TexWin pixel shader: off
- VRam size: 64 MBytes
- FPS limitation: on
- Frame skipping: off
- FPS limit: 60.0
- Offscreen drawing: 2
- Framebuffer effects: 3
- Framebuffer uploads: 2
- Scanlines: off
- Mdec filter: on
- Screen filtering: on
- Shader effects: 0/1
- Flicker-fix border size: 1
- GF4/XP crash fix: off
- Game fixes: off 
put frame skipping on, iirc there is an auto skip option (Autodetect frame limit/skipping), that should speed it up, if not try a different plugin, his Direct3d one should be alright.
also check your display settings in windows, under the ATi tabs, make sure you dont have Anti Aliasing or anything on, the Radeon 9000 is rubbish, so if all else fails, get a new graphics card ;p
Avoid using frame-skipping.
They can create more problem with the frontbuffer accesses than solving it.
It is true the Radeon 9000 is rubbish, but with his specs, his computer should be able to emulate Gran Tursimo with full frame-rates and without any significant problems except for slowdowns when ePSXe attempts to access the CD-ROM drive.