Need help with video card

1. power supply?

2. PCI-E x8 or x16 slots? The 2.0 card should be backwards compatible with 1.1 slots. Most GPUs today are fine with PCI-e 1.1 slots

3. CPU and memory?

The cpu is Intel Pentium 4 3.20 GHz. 3 gb of kingmax ddr2 ram, PCI-E x 16 slots and a Thermaltake Purepower 420 watts power supply.You say that the 2.0 cards are backward compatible with 1.1 slots but is a 1.0b slot the same as a 1.1 slot?

you may go for radeon 4670 512mb ddr3. this card will run fine in your pc. if you want more, you can buy -

1. radeon 4830 or geforce 8800gt
2. radeon 4850 or geforce 9800gtx+


but you need to upgrade your power supply to run stage 1 or 2 graphics cards in your system. and you can run up to stage 2 cards in your pc. though these are pci express 2.0 cards, you can run the surely.  

upgrading you processor to pentium dual core E2200 (2.2 ghz, 1mb cache, 800 fsb) will be a great upgrade.  

^ Hi there, how many amps are on that PSU? If you have around 20 total on 12v rails then you have enough. (I've heard the 9800GTX models draws anywhere from 12 to 18)

+1 to Cryvidia's GPU suggestions