New build for game dev

[Aono]

Building a new PC for gaming and game development. I can spend around £600-700. I can reuse any bits from my old one:

Gigabyte P35C-DS3R
Nexus NX-5000 R3 530W PSU
Intel Core2Duo E6850 3.0Ghz
4GB DDR2
Radeon HD5770
Samsung SpinPoint F3 1TB x2 (one for backup)
LG 24" LCD
Antec Dragon case

I need to be able to run lots of resource hungry programs at once and tab between them quickly (eg, photoshop + visual studio + modelling program + game running in background + browser with a million tabs open). Should I get 16gb for that or is 8gb enough?

The only limitation I have is I need a nvida card for CUDA.

So far I'm thinking something like:
GeForce GTX 660TI - £250
Intel Core i5-3570 3.40GHz? - £160
Which motherboard? Maybe Gigabyte Z77-D3H? - £80
What memory? Maybe GeIL Black Dragon 16GB? - £60

Can I reuse my old PSU?
Can I fit in a SSD somehow?
Anything else I should consider?

I'm not in a huge rush so maybe I can wait for the new year if prices will be better/new stuff will come out but I don't to wait until Q2.

Thanks for any advice.

 

[Aono]

Forgot to say I use an old 19" Samsung as a second screen. I will probably upgrade that to another 24" at some point but that's not included in the budget.

Review of the PSU I've got:
http://www.silentpcreview.com/article1012-page1.html

It's very quiet I'm just wondering if it's powerful enough.

 

[Aono]

Thanks for the suggestions and help. I can workout some combination of the above I'm happy with.