Based on these system specs for the game from the support site, NO GuildWars is Not a 'DX9 Capable Video Card Required' Game. No idea if having the extra Shader support that DX9 provides would help or not.
Minimum System Specs:
Windows® XP/2000/ME/98
Intel® Pentium® III 800 MHz or equivalent
256 MB RAM
CD-ROM Drive
2 GB Available HDD Space
ATI™ Radeon® 8500 or GeForce 3 or 4 MX Series Video Card with 32MB of VRAM
16-bit Sound Card
Internet connection
Keyboard and mouse
Recommended System Specs:
Windows XP/2000/ME/98
Intel Pentium III 1 GHz or equivalent
512 MB RAM
CD-ROM Drive
2 GB Available HDD Space
ATI Radeon 9000 or GeForce 4 Ti Series Video Card with 64MB of VRAM
16-bit Sound Card
Internet connection
Keyboard and mouse
Guild Wars is DX8 game. Forcing it to DX9 wouldn't help since the game wouldn't take advantage of the DX9 features anyways. Just wait for graphic engine upgrades, if there's any coming. Maybe at a later time.