Best video card for my PC....Plz help

I was told that the ATI Radeon HD 5770 512 MB was the card
to purchase for my system. Is this the right card? I'm looking to spend $150-$175 total, with no PSU upgrades. Here is my
system :
Mainboard : ECS Nettle2
> Chipset : nVidia 6100V
> Processor : AMD Athlon 64 X2 4800+ @ 2500 MHz
> Physical Memory : 3072 MB
> Video Card : NVIDIA GeForce 6150SE nForce 430
> Hard Disk : WDC (320 GB)
> DVD-Rom Drive : TSSTcorp CD/DVDW TS-H653L SCSI CdRom Device
> Monitor Type : Hewlett Packard HP w2207 - 22 inches
> Network Card : AN983 FastNIC PCI 10/100 Fast Ethernet Adapter
> Operating System : Windows Vista (TM) Home Premium Home Edition 6.00.6002 Service Pack 2
> DirectX : Version 11.00
Or should I purchase the 1 GB? I just would like a second opinion to make sure that is the right card. Appreciate any and all help.
Thank you

Welcome to DEV.
What is the main purpose of this upgrade ?
Do you game ?
What games do you play and at what resolution ?

Personally I think you will see some bottlenecking from that AMD 4800 / 5770 combo.
You also state you don't want a PSU upgrade...what PSU do you have.
The 5770 at full throddle uses approx 284 watts
Have you checked out the PSU Calculator to make sure you have enough wattage.
More important than wattage is the amperage on the 12 volt rails.
And how old is your PSU

ATI Radeon™ HD 5770 System Requirements
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
I hate to see you plug it in and have the power supply take out something on the mobo redering it useless.
Happened to me just about 5 weeks ago...got a GTX 260 for x-mas went to plug it in and my PSU (650 Antec) popped
It took out my PCIE slot.
In 2D I was fine,couldn't play a game for more than 5 mins without crashing...lucky I had a spare mobo laying around...haven't crashed since the new Mobo.
Let us know how everything works out

I am upgrading to play games..like Dragonage, some MMO's. at 1680 x 1050 res.. I dont wanna play Crysis.. just some good decent games and not lag out all the time. I really want to be able to play FF XIII and FF XIV when they arrive.. Playing a couple of f2p's right now..and they are nothing but lag.. Would like this to last until im able to get a whole new system.. a year maybe? Im not tech savy or even slightly knowledgeable so you lost me at bottlenecking. Sorry.

Bottlenecking in laymans terms is like having a 4 lane hi-way converge down to a 2 lane hi-way
There gonna be some slowdowns.
So basically it would be your CPU that would be the slowdown.
You could try playing @ 1280 x 1024 and maybe turning down AA and AF
(AA)Anti Aliasing: The feature that removes the Jagged lines that are created when you draw a diaganal line on a screen. It removes the jaggies by adding a slightly darker shade next to the pixel creating a blurred effect that makes the image more smooth.

(AF)Anisotropic filtering: Is Texture filtering of objects that are far away and usually tilted at an angle the further object is blurred slightly to help the effect of distance.

Thank you very much. I appreciate you taking the time to explain to me.

It shouldnt i guess but just to be on the safe side why not upgrade ur PC's specs a lil.. since u r a heavy gamer, upgrading ur PC would be to ur advantage...