If you already have hynix, then stick with it if you can find it at a good price. Your dell doesn't have any bios adjustments for the ram speed; so no matter what brand you buy, the speed will remain the same. I use craigslist to shop for used dell products, so you may get lucky and find some that match your current ram.
I'm really new at upgrading computer stuff, so correct me if I'm wrong: I saw a thread in TH that said that their Studio 540 could handle 800 mhz DDR2. Would you recommend me look for those types or stick with 400 mhz?