I want to build a new system. I've done some research but i can't choose between a AMD 3700+ and a p4 630
Depends for what purposes you are going to use the system.
The P4 630 is 90nm and is not the best class of singlecore P4, the 631 is the same but is 65nm and produces less heat. Anyway this does not metter if you are not overclocking your systems. It has nice performance/price factor compared to the others from the 6xx series. If you plan to upgrade your system in near future, than buying a motherboard that supports Conroe and apropriate DDR2 will make the things easier for you. The 630 will overperform the 3700+ for rendering and office use, while the 3700+ is better for gaming, encoding and multimedia. If I was you I will probably buy Opteron 146 Venus 2.0GHz. It costs a few dollars more than the 3700+, but is much more overclockable and stable.
See some benchmark comparisons(www.tomshardware.com) and compare the overall system price including motherboard and RAM. Make comparison not only beween those two CPUs, but include some other chips that can fit in your new system budget. If you plan to overclock, than try to find some oc-ing expiriences and reults and again use the peformance/price method to determinate which system is better for your purposes.
The 3700+ is the better choice. It will outperform the 630 in most areas, and in the unlikely situation it is bested, the difference is marginal and for the most part unnoticeable - a small price to pay for the lower power consumption and heat output.
Keep in mind that with the 630 you're going to want to buy an aftermarket HS/F as the Intel supplied one is awful.(loud & inefficient) Newegg lists the 3700+ at $235 and the 630 at $167, but most of the the money you'd save on the 630 will go to the HS/F. I personally built a 630 based system a few months back for personal use and have been very pleased after replacing the stock cooler.