- First of all, buying a used GPU is more of a gamble as you don't know how it has been used (or abused) ... we have high temps here and most people don't replace the TIM after one season (it dries out) ... result is reduced lifespan of the gpu
- Secondly, you need to determine the version of your pci-e x16 slot ... if its version 1.1, then, you are likely limited to v2.0 spec cards (though there is backwards compatibility but you never know ... there also might be conflicts with HDMI audio with mobos for P4)
- Then, you need to decide on a GPU within limits of your power supply (if you don't intend to update it as well)
- I would suggest that either you spend the 10K to upgrade your entire system by getting a used branded system with good enough integrated graphics and start saving up for a newer GPU (sell or trade in your old system) or you may try to see if you can get a new AMD HD7750 1GB GDDR5 (better for gaming, no extra power needed but pci-e 3.0) or a new nVidia GT 640 Rev 2 GK208 GDDR5 (better overall, very low wattage and pci-e 2.0) working in your current system
Note: Newer low wattage system and gpu may offer good savings (the expensive units cost around Rs. 16) if you use your system for longer periods!