Which Drivers do I need for Vista & where can I get them ?

I have a Dell Optiplex GX 620 System that came with Win XP PRO SP2.

I want to install Vista ( Ultimate ) 32 bit

Which drivers do I need & where can I get them ?

Do I need new BIOS ?

I am not a techy but

Dell says that Vista will not work on this machine which is strange.

Machine Specs are :

Intel Pentium D 3.0 GHz Dual core Processor

2.0 GB DDR RAM

250 GB HDD

256 MB ATI Radion X600 Video Card capable of running two monitors

Broadcom NetXtreme 57xx Gigabit Controller

Conexant D860 56K V.9x DFVc Modem

SoundMax Integrated Digital Audio

NEC DVD+RW ND-3530A

SONY DVD-ROM DDU1615

Please can someone give me a step by step as to how do I install the drivers too.

Thank You in Advance.