Upgrading hard drive vs. video card- need help

i am in the process of choosing a new upgrade for my gaming computer. first, here is my current setup:

- Asus A8N-SLI Premium motherboard
- AMD Athlon64 4000+ CPU
- 3GB DDR400 PC3200 RAM (1x1GB, 2x 512MB)
- 2x BFG GeForce 7800GTX 256MB Overclocked video cards
- 120GB SATA150 7200RM Maxtor Diamondmax HD
- Generic 450/500W PSU (can't remember which it is)

i am trying to decide between two upgrade options and i'm not entirely certain which option to go with in order to get the best performance boost for playing games (mainstream games).

Option 1: upgrade my current hard drive to a Seagate SATA300 7200RPM 500GB HD

Option 2: upgrade my current set of video cards with a Diamond ATI Radeon 3650 1GB video card

My assumption is that the video card upgrade option would be the better bet- even if the card is a low end one i think it still beats out the combo i have right now in terms of performance.

Can anyone provide some suggestions or advice on this?

Any help would be greatly appreciated.

Thanks.

sorry i forgot to include the memory specs for the video cards (i have edited my original post to reflect that). they are both 256MB cards.

i am typically playing at a resolution of 1024x768- 1280x1024 if i can safely pull it off without noticeable impact on the game.

budget doesn't play into this decision because i have a gift card for a local tech shop and this is the best card they have available within the price that i can afford with the gift card.

thanks.