Computer running slow during game, new parts didn't help.

Okay, this is the issue my brother is having:
He says he can't play Tiger Woods PGA Golf 2004 because the game slows
to a crawl. It appears the longer the computer is on the more sluggish
the game gets.
He doesn't play any intensive games, but this is why we had upgraded
the computer last week.
We settled on the following components:
JUST PC JPC767P White Steel ATX Mid Tower 450W Power Supply - Retail
POWER SP|485W(P4)EG495P-VE SFMA - Retail - JUST UPGRADED TO THIS
MSI 865PE NEO2-V 865PE 478 MB - Retail - JUST UPGRADED TO THIS
Intel Celeron D 330 2.66Gh Prescott 533MHz FSB 256KB L2 Cache Socket
478 Processor - Retail
CORSAIR 1Gb x 2 D400 VS2GBKIT400C3 Memory - JUST UPGRADED TO THIS
POWERCOLOR R96-LD3 Radeon 9550 256MB DDR AGP 4X/8X Video Card - Retail
Maxtor DiamondMax 10 6L200R0 200GB 7200 RPM 16MB Cache IDE Ultra
ATA133 Hard Drive - OEM x 2
Windows XP Pro.
Norton's Internet Security and SystemWorks both 2006 versions

The only thing we haven't changed is the case and the video card.
Everything else has been purchased within the last 2-3 months. I guess
it's the video card, but I wanted opinions about this issue, and more
importantly, recommendations on a replacement video card, that can
handle video intensive games in case he wants to eventually play them.

This will mean alot to us, as we play the Tiger Woods game together.
Thanks for anyone's help!!!