4850 512mb vs 4850 1gb vs 9800GTX+ ?

[icyicy]

Hello, I am planning to build a PC and I have it all figured out, except the graphics card.

I am looking for something in the 160-200Euro range and I've been mainly looking at the ATI HD 4850 and the Gforce 9800GTX+ but I have some questions, I was hoping you could help me out here.

First, let me tell you about the PC I was thinking about, in case that helps:

Case: THERMALTAKE CASE VD2000BNS MATRIX BLACK
PSU: THERMALTAKE W0101 TR2 POWER 550W
CPU: INTEL CORE 2 QUAD CORE Q6600 2.40 GHZ LGA775 - 1066 FSB - BOX
Motherboard: ASUS P5Q PRO
RAM: CORSAIR XMS2 DHX DDR2 4GB (2X2GB) PC2-8500 (1066MHZ) DUAL CHANNEL KIT
HDD: WESTERN DIGITAL 640GB WD6400AAKS CAVIAR SE16 SATA

So here's the prices from the shop which I'll be buying the graphics cards from:
MSI R4850-T2D512 512MB PCI-E RETAIL for 166E
ASUS EAH4850/HTDI 1GB PCI-E RETAIL for 199E
GAINWARD 9405 BLISS 9800GTX+ 512MB PCI-E RETAIL for 166E
ASUS EN9800GTX+/HTDP 512MB PCI-E RETAIL for 189E

And now the questions start.
First of all, how big is the difference between the 512 4850 and the 1gb one? I have only been able to find information and reviews for the 512mb one.
Secondly, why is the ASUS 9800GTX+ card more expensive?
Thirdly is the physX on the Gforce cards going to make a big difference?
I don't really plan on using Crossfire or SLI if that helps.
Also, if you see any problems with any of the other pieces (e.g. do I need a bigger PSU, should I pick a different mobo if I go with the gforce, etc) feel free to tell me.

Thanks for your help.

 

[dirtmountain]

The HD4850 512, unless you're running at very high resolution.
Good quality RAM DDR2 800 that runs at 1.8 volts