MultiUpgrading Help. (Newb etc.)

Ok, I'll start off by being totally honest. I have 0% experience with upgrading Hardware parts, I guess I'm just used to working with the software side of pcs. So if I'm failing like about to buy something which is absolutely useless don't hesitate to tell me.

First off, about 3-4 years ago I got a Alienware PC. (Yeah before you start, I'll admit overpriced, over hyped PC I totally agree with you.)

I'm not sure if this is even the right section, but what I'm looking to do is upgrade the main parts of my PC. That being:
GPU / Graphics Card.
CPU
PSU
RAM
And if it's a must Motherboard.

Things like my soundcard, and Hardrives apart from a few problems I can put up with. After all I'm a student, so upgrading everything at the same time isn't really possible. Saved and such to get this far lol.
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Current PC:

OS
Windows XP Professional Service Pack 2

Monitor:
Samsung SyncMaster 19inch.

Graphics:
NVIDIA GeForce 7800 GT (2x) (SLi is enabled)

Motherboard (Pretty sure it is.):
ASUSTeK Computer INC. A8N-SLI DELUXE 1.XX

Harddrives:
HDS724040KLSA80 [Hard drive] (400.09 GB) -- drive 0
HDS724040KLSA80 [Hard drive] (400.09 GB) -- drive 1
c: (NTFS on drive 1) 400.08 GB 290.51 GB free
d: (NTFS on drive 0) 400.08 GB 228.33 GB free

CPU:
2.20 gigahertz AMD Athlon 64 X2 Dual Core

RAM:
1024 Megabytes Installed Memory
Slot 'A0' has 512 MB
Slot 'A1' is Empty
Slot 'A2' has 512 MB
Slot 'A3' is Empty

PSU:
I don't actually know if I need to upgrade this, due to the fact I'm not sure if the new parts will demand that much more power. That and the fact I have no idea how you can calculate the PSU's specs.
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Future PC (Or what I hope will work.):

OS
Windows XP Professional Service Pack 2
Don't feel like I should move to Vista yet.

Monitor:
Samsung Syncmaster SM245B 24" Widescreen LCD Monitor
(24", 1920x1200, 5ms, DVI - MPN: LS24HUBCBQ)

(Would my future graphics card be able to handle this?)

Graphics:
Asus EN8800GTX/HTDP/768M GeForce 8800 GTX Video Card
(768MB, PCI Express x16, Dual DVI - MPN: 90C3CFP0PUAY00T)

Motherboard:
Is it so horribly outdated, that it's a must to upgrade? I know it being outdated will slightly effect things, but I'm talking about effecting it so much to extent of not being able to see/use the new graphics card or RAM.

Harddrives:
Once again, no real plans to upgrade.

CPU:
What would you guys recommend. Quad core? Higher up GHz Dual core? I'd stay on AMD to be on the safe side.

RAM:
I don't think you can have 3gb's of RAM? (Correct me if I'm wrong.)
I'd either end up buying 4x sticks of 1gb
Or just 1x Stick of 1gb. (Unless you can't mix 1gb with 2x512mb?)

Model: ASUS MOTHERBOARD A8N-SLI/1-C15

PSU:
If I had to upgrade my current one, which I have no idea if I should. Would this cover up to today's type of PC:
Corsair CMPSU-620HXUK 620W Power Supply w/ Modular Connectors
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Sorry to put up such a long post, but I thought I should include as much information as possible.
There's a few questions in there, which I'd need advice on.

Like I said, I'm a hard ware noob. But the main reasons I'd like to upgrade are things like, slugish speeds, Photoshop Errors: "Could not open the document not enough RAM."*, Games like Teamfortress 2 having 30 fps when playing on Medium graphical settings.

*Believe me I've tried exiting unnecessary apps and processes, didn't do me any favours.

Thanks for your time.

Thanks.

So there maybe some hope yet, since I instead of spending £100 on a PSU I need a motherboard instead.
But yeah, I'll still be buying a CPU and RAM and GPU. And obviously the motherboard. I'll probably take your advice on the RAM. Since I need to upgrade it but won't be able to afford DDR3. Not sure about CPU though. And obviously I've got my sights set on that Graphics card.

If you could be so kind, what specific:
motherboard would you recommend for my situation?
CPU would you recommend for my situation?

Thanks again, I would have probably bought a new power supply, with out realising it's most likely not needed.