To upgrade or not?

[Chris56]

Hi all,

I currently run the following PC (have owned for 3 years) and is becomming increasingly slow, particularly on startup:

AMD Athlon 64 X2 5600 (2.9GHz dual core)
4GB Ram (showing 3.25 however)
Nvidia 512 MB graphics card
250 GB Hard Drive (145 GB used)
Windows XP home (SP3)

To maintain performance, I regularly do the following:
Defrags
Cleanups
Virus Scans
Reduce number of programs on startup to a minimum.

My real question is, what should I do?????

I am considering any of the following:

Buy a new PC (well, xmas is comming!!!!)
Add more RAM (motherboard will take up to 8GB)
New Hard Drive
Upgrade to Win 7

My uses are mostly word processing, internet, streamed videos and some games (though I am not really into the most recent games or building a super performance PC)

Are there any experts out there who can offer any advice.
If changing the PC, I have been considering going for an Intel I5. I understand that they have an integrated graphics controller. Is this any good or would you recommend disabling this and fitting my existing Nvidia card.

Your advice is much appreciated