Which graphics card to buy?

You have a fairly old system by today's standards. Unfortunately, it appears Intel no longer offers online support / documentation for your motherboard or chipset anymore.  

The processor you have in your system is also fairly slow for gaming purposes. Although a dual-core processor is adequate for many games still, the 1.8Ghz speed on yours is going to limit you quite a bit.

Also, 1GB of RAM is going to severely hamper gaming performance as well. Even if you're running Windows XP which uses less RAM than Vista or Windows 7. I don't know what budget you have available, but honestly a new PC is probably in order to play newer titles like Assassins Creed 2 in particular.

The good news is though, at 1280x1024 you don't need a very powerful video card for most of what you want to play.

Can you determine if this computer has a PCI-E x16 expansion slot? It may say on the original owner's manual, or on a sticker on the PC somewhere. You could also tell us the Brand name and Model number of the PC too. I might be able to look up the actual motherboard specs through their website. Intel doesn't offer any technical details on their site anymore.

Maybe nVidia GT240 DDR3 or Radeon HD 5670 without any other upgrades.

hd 5670 is best for him,he needs to overclock pc and then add 1gb more gb ram and then upgrade to win 7

ATI 5670 yes, Overlock CPU yes, Add RAM yes, but why do they need Windows 7 exactly? I don't see how that's at all a requirement.

well thanks for your all suggestions .here are some more detail you may want
i use win7 extreme edition
i m pasting link which has my mb specifications acc. to which i have" One PCI Express x16 bus add-in card connector"
http://downloadmirror.intel.com/15117/eng/D945GCCR_Tech...
further my budget is around 6000rs. for graphics and acc. to when i checked prices in my country(ebay india) i found 9800gt best choice 5670 costs around 7000 in it.
also i havnt got any positive feedback of ati graphics card.i think nvidia offers better product support.
currently my psu is of 450watt
now last doubt: can anyone tell whether my mb will support 1gb ram stick ram of freq. more than 333 mhz?

well thanks for your all suggestions .here are some more detail you may want
i use win7 extreme edition
i m pasting link which has my mb specifications acc. to which i have" One PCI Express x16 bus add-in card connector"
http://downloadmirror.intel.com/15 [...] odSpec.pdf
further my budget is around 6000rs. for graphics and acc. to when i checked prices in my country(ebay india) i found 9800gt best choice 5670 costs around 7000 in it.
also i havnt got any positive feedback of ati graphics card.i think nvidia offers better product support.
currently my psu is of 450watt will i be able to play games at med settings nd around 1024*768 res with 9800gt?
and also can i overclock my processor?my mb is of intel so i doubt if they support overclocking.

Just look at the memory specs on page 16 of the manual you linked above.

Here is a link comparing performance stats for the HD 5670 and 9800 GT.

http://www.tomshardware.com/charts/2010-gaming-graphics...


Note that the HD 5670 in the chart has 1 GB of memory. Some models come with only 512 MB and should be avoided if possible. Also make sure the 5670 has GDDR5 memory. I think they all do but it is worth confirming on any model you are thinking of getting.