I am upgrading my current PC. As it stands it is this:

8800 GTS 320MB
AMD 5000+ X2 ~ 2.6Ghz
2 GB DDR2 800
NForce 4 Chip
700W PSU

I would like to purchase:

ATI Radeon 5770 1GB (With the eye to purchase another one in future for cheaper)
AMD Phenom II 555 Black Edition (With 2 cores but 1 guaranteed unlockable core, possibly 2 if I am lucky as my dad was when he got his)
2 GB DDR3 1600
ASUS Motherboard that supports core unlocking and has a 790GX chipset supporting Crossfire.

It would suit me to purchase the GPU first for £130 and then upgrade the rest at a later date, if there is any point in that. I am not sure the bottlenecking effect will be of my other existing components so I would like advice on this.

Another option is to purchase the Motherboard, CPU and RAM and stick with the 8800 I have until a later date but my main priority is just to be able to play New Vegas and Deus Ex 3 at reasonable settings when they come out.

The total of the 4 components is £365.08 Inc Delivery.

It would require £231 for the set up minus the GPU and £134 for the GPU on it's own.

My argument for the 5770 is that it is a good card (My dad has one) and it should come down in price when I start feeling the need to upgrade and if I decide to crossfire it after some time I will have spent a lot less than the £250 for a top of the range card with good longevity (Say the 5870 costs a ridiculous amount).

A factor to take into account is my screen resolution that is 1920x1200. I took my monitor downstairs and tested Crysis Warhead, Red Faction 2 and Just Cause 2 on my dad's rig and it seemed to be ~ 30fps for all tests on high settings for his 5770/555/2Gig system set up.

Let's hear your nerd wisdom then!

Are you sure you only want 2GB? I've definitely gone over that with certain games before.

That's something I can upgrade for £50 when I need to cross that bridge. Emphasis on low budget here. I've never had any problems before.

You'll probably notice the biggest difference by swapping out your GPU first and then worrying about the other components, although the GPU will probably be bottlenecked it should still be a significant upgrade.

Can I have your old stuff

Sure if you'll give me a few bob towards my new stuff (I have all the boxes etc still).

Edit: Bought the 5770, the 8800 is up for grabs.