Max number of hard disks for 450 watt psu

I've got 2 internal hard disk and 1 external hard disk (powered externally). I'm planning to get myself another internal hard disk, most likely a WESTERN DIGITAL Caviar Black 1.0TB. My two other internal hard disks is a Samsung HD502IJ and a Hitachi HDS 721616PLAT.
I'm using a Nvidia 9600GT graphics card (overclocked about 10%) and a Intel Core 2 quad (Q8200) cpu (overclocked about 6%). I'm currently using a cheap 450 watt PSU.
I'm a total noob when it comes to PSU but I do understand that higher wattage PSU are better when I did my research on the internet. But nothing answers my question on the hard disk part. So what I'd like to know is do I need to get a better PSU to provide more juice or is my current PSU is good enough that I can add in one more hard disk without worrying about not having enough power.

Thanks for the quick reply. Since it was requested and I want to know if my current PSU can handle the load, here's my system's specs:

CPU: Intel Core 2 Quad (Q8200) -6% Overclocked
Motherboard: ECS G31T-M7 (cheap one, I know >.> )
RAM: 4GB
Graphics card: NVIDIA GeForce 9600GT (dual screening with this card. 10% Overclocked)
Network: D-Link DWA-547 Rangebooster N650
OS: Windows Vista SP2 32bit. (changing to Windows 7 64bit soon)
C & D Drive: Samsung HD502IJ (partitioned to C and D. SATA)
H Drive: Hitachi HDS 721616PLAT (PATA)
I Drive: Western Digital 5000AAV (external hdd)
F Drive (DVD): SONY DVD RW DRU-8455
Others: Internal USB card reader
7 x USB Ports, all in constant use
Western Digital WD400 HDD. (in the PC casing but not attached to any cable for the moment but might attach them soon)

I still have other 2 more hard disk around (all filled up) and I'm thinking of connecting them as well. Someone recommended me to use an external enclosure, but I had bad experience with them as those enclosures either crash my drives or its adapter suddenly not giving enough juice to spin the disk.

Thanks^^

I forgot to add in the PSU oops. I'm not sure what is the brand since there's no brand label but the model is PSIV-400-1 according to the attached sticker. It came together with my Asus TA-861 Vento A8 Chassis (old skool XD). A simple google showed me that it's some generic PSU. Also, I've mistaken my PSU wattage. It's actually only 300 watts.

edit: I've decided on a Cooler Master 650 watts PSU. Installation went well (did cut my thumb though. blood got on the black matte casing T_T) and I'm starting to like it. The previous PSU made lots of buzzing sounds but this new PSU is running silent. I used a PSU calculator which gave me a result of 385 watts, but with possible upgrades in the near future, I went with more wattage. I've added in another drive (the others still not in yet) but ran out of space. Need a 5.25" to 3.5" bracket for the rest. Well, rearranging the cables to allow more airflow is a pain though...