If I were you I'd opt for the Crucial 2x2gb 4-4-4-12 kit.
Newegg.com - Crucial Ballistix 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model BL2KIT25664AA80A
The timings are better. It's more overclockable. It's less expensive, and it has free shipping.
Timings mean a lot with memory. DDR2 800 5-5-5-18 is not very good. The kit you chose to replace your current kit only has the advantage of capacity. This will help, but not as much as you think. You won't notice any increase in FPS at all. In fact you might notice a drop in FPS in games, and generally slower performance.
However, the kit that I recommended will increase performance, overclockability, and general responsiveness. Not only this but it will save you a few dollars, and it's still 4gb.