Deciding if i should purchase a video card

[donutpink]

Hello, I am relatively new to this tech site. I would like to ask some advice in purchasing a video card for my computer. This computer I have bought one year ago came with not so good quality integrated video card.
I was planning in buying the Sapphire Radeon HD5770 video card from my computer. I understand buying a video card involves many factors. I recently bought a ASUS 23inch LED monitor with resolution up to 1920 by 1080. Previous from buying this monitor I had a 19 inch gateway monitor that i would usually play starcraft2 on low settings with very minimal lag. Now as I switched to the new ASUS 23inch monitor, I have noticed that it lags. By lag, when i move my screen it is not smooth but a little choppy.
The reason I am hesitant to buy the video card is that the video card might not reduce lag when I am playing games such as sc2 because of my computer and the resolution of the monitor. I am afraid the video card can "bottle neck" because of the specs of my computer.
I am also planning to dual screen with my old 19inch monitor that is why i chose the Sapphire Radeon HD5770

Well now here are my questions

Will buying the Sapphire Radeon HD5770 video card reduce the lag of games while being able to play on high resolution?

Are there any other less expensive video cards that I can use so the video card will not "bottle neck" and maximize the potential of the video card, while being able to play on decent resolution on my ASUS 23 inch monitor?

Any advice about my problem of deciding in purchasing this video card would be great
I do not know all the specs to my computer but this is the most I know.

AMD Athlon II X4 630 Processor 2.80GHz
4.00 GB of RAM
Windowsn 7 64bit

I appreciate all for reading my dilemma and answering my questions. Thank you

 

[MindoverMaster]

Get the XFX 5770. Lifetime warranty.
Newegg.com - XFX HD-577A-ZNFC Radeon HD 5770 (Juniper XT) 1GB 128-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card
$104 after MIR.

What PSU do you have?

 

[donutpink]

That is another problem i did forgot to mention in before. The max output for my PSU is about 250W. I have another PSU from CoolMax model V-400 from my old computer. It says here that, the max output is 348W. I have read some reviews on the internet about this PSU CoolMax v400, and people have mention that it was a bit noisy. I was also thinking about investing in some money to purchase a new PSU or to just stick with the CoolMax v400.

I do not see the the MIR for the XFX 5770 on newegg. Why should i purchase the XFX 5770 instead of the Sapphire Radeon 5770?
Also would the XFX 5770 help solve my dilemma?

 

[MindoverMaster]

Both the Sapphire sand XFX use the same amount of wattage. You have a 250W PSU? That's your problem. You don't have enough power.

Why is the XFX better? Try matching up 2 year warranty with lifetime warranty. BIG difference.

 

[thesilversavers]

Mindover is right.

XFX is better. I'm using a HD6990 from XFX and I can do the bios overclock without voiding my warranty.

Are you sure a HD5770 is enough for you? If you have the budget I'd recommend a HD6970 or something around that specs.

Also, for you PSU, you'll risk blowing your PSU and the whole computer if you stay with the 250W or 400W. To be safe, go for the Earthwatts 750W by antec. I've used their 650 watt one in the past and it powered my overclocked EVGA GTX 285 with no problems. Its about 95 dollars.

Are you going to play games like Crysis? To run it on the 1080p screen you would need at least a GTX 570. If your budget allows for it, go for the close to top end GPUs

 

[MindoverMaster]

Earthwatts are awesome for the price. I have their 380W brand on my home server. Been with it for at least 2 years. No problems.