Upgrade or not to upgrade

[Kendo Nagasaki]

As the title suggests, is it worth upgrading my current computer (see below). I use it for a variety of applications including, photoshop, CAD, web design, 3D work, etc and have found that it is beginning to get a bit sluggish sometimes.
Maybe i could do with a format to help increase performance - who knows!! but can anyone suggest anything that could bring my current rig up to speed? Dual-core springs to mind.
I'm looking into upgrading the monitor in the new year (BenQ 24" possibly), so maybe that is something to bear in mind.
Also, my current computer isn't overclocked, cos i haven't a baldy in this area.

My current computer specification:

 DFI LanParty UT NF4 SLI-D (Socket 939) PCI-Express Motherboard

 AMD Athlon 64 3700+ San Diego 90nm (Socket 939)

 Zalman CNPS9500-LED Aero Flower CPU Cooler

 Corsair 2GB DDR XMS4000PT TwinX (2x1GB) CAS3

 Leadtek GeForce 7800GT 256MB GDDR3 VIVO TV-Out/Dual DVI (PCI-Express)

 Western Digital Caviar Special Edition 250GB 2500KS SATA-II 16MB Cache – OEM

 Lian-Li PC-61 Black Aluminium Midi-Tower Case (No PSU)

 Hiper HPU-4B580 Type R 580W Modular ATX2.2 PSU – Blue

 NEC ND4551 16x16 DVD±RW Dual Layer ReWriter (Black)

 Enermax Aurora brushed aluminium keyboard

 Icemat - Black 2nd Edition

 Microsoft Laser Mouse 6000

 Creative Sound Blaster X-Fi Xtreme Music

 Acoustic Energy Aego M 2.1 Speakers (Black)

 Dell 19” CRT Monitor

Maybe i don't need to upgrade at all, just thinking out loud.

Kenny

 

[Kendo Nagasaki]

It looks more than likely that'll be upgrading to a 24" screen but before I order can someone have a look at my current spec and see if it'll be ok to handle it.

If I was to move across to the Intel Core 2 Duo, would I be able to keep everything apart from the motherboard & processor (obviously)? Is the RAM ok?

Would it even be worth upgrading the graphics card?

I have about £550 to spend.

Cheers.

 

[Kendo Nagasaki]

Oops, double post