He could get away with running an HD4670/HD5750/HD6750 on a 400W, the 4670 doesn't require a 6pin the others will come with adaptors for the 6pin, I ran a Sapphire HD4670 in an Athlon II x3 system and it ran crysis at high on a 1366x768 monitor.
But I would definately reccomend getting another 2 gig stick of ram, so atleast you can run in dual channel mode. and if you can afford it, an 80+ certified PSU wou;dnt be a bad idea to future proof your self.
I would recommend to add another 2GB memory, and buy a AMD Radeon HD6670 graphics card with 1GB GDDR5 memory. You will get the card at around USD 100 and it would give you exceptional performance in 1366*768 resolution. And also you don't need to bother about spending for a new PSU with 6 or 8 pin PCI-e power connector.
32-bit OS cannot use full 4GB memory, but at least it would increase 1.25GB than the previous.
2gb ram + 5670/6670, that will run games well and is all you need for that resolution, you also wont need to upgrade the PSU. the only thing missing is a quad core CPU, some new games like COD black ops and Crysis 2 dont run well on dual core CPU's. Most games will run ok though.
i'm lukng around $100-120, so i have 4 choices now i.e 5750/6750 or 6670/5670...if i'm not wrong x7xx is better than x6xx and 6000 series is new that makes going for 6750 a better option ...can i make that assumption
your power supply wont have enough power for the 5750/6750. forget it. get the 6670/5670. and just fyi the 6750 is a re-badged 5750, same as the 6770/5770. the 6670 is slightly faster than the 5670, its not just a re-badge. You MUST upgrade the ram also, put another 2gb in.