Dell XPS 420: Time for a change...

Firstly having trawled the internet I'm seeing so many conflicting pieces of advice about what can and can't be upgraded in this PC. Firstly my current specs:

Intel Quad Core Q6600 2.40qhz

Crucial Ballistix Tracer 2 GB Memory - DIMM 240-pin - 800 MHz ( PC2-6400 ) x 4 (8 gig in total obviously)

NVIDEA Geforce 8800GT

3 x Samsung HDD's (original 2 x 250gb and 1 x 1TB)

Vista 64bit

375W PSU

Now, firstly I've had the same issues that a lot of owners have had in that I get intermittant freezes for no apparent reason. I've got avast and adaware installed and maintain my PC using advanced system care pro. I believe the problems are connected to the original HDDs? I'm not running any RAID setups. 

Basically I want to know what I can do to stabilize things a little. Should I get a new mobo to increase performance? Maybe a new case and transfer the memory, cpu and graphics card over? I don't usually play many graphics intensive games, I'm more of a football manager man myself but I can't help but feel I'm not getting the most out of my processor.

I'd like some recommendations if possible for two options. I'm not looking to break the bank but maybe a budget of say 150 - 200 quid?:

1) Keeping the case but put some better components in it

2) Replacing the case and gutting the XPS 420 rig taking the best bits out and coupling them with some new parts

Apologies for the blatant newbie post here. I know a fair amount about PC's but its been a while since I upgraded one and I'm a bit out of touch with what options I have available to me. If anyone can offer any advice whether it be from past experience of upgrading their own xps 420 rig or just advice in general I would be most grateful.

Cheers guys I hope someone can help me!

mcbgun