OK so i want to upgrade my Dell

Dell Dimension 4300
Pentium 4 CPU 1500MHz
1.50 GHz
256 MB of RAM
NVIDIA GeForce2 MX/MX 400

I want something better and a little bit faster. NOT too expensive.
I want LFS to run perfectly on almost best graphics and speed.
Is it even worth it?
Or should i just save up some money and buy a new machine? Or build my own?

If you have enough money to build a new system then by all means do. Parts that you may be able to reuse would be PSU (how many watts?), case (could be that DELL doesn't use standard mounting points) and HD(s).

The best bang for the buck components you can get at the moment would be AMD Opteron 144 or 146 (if you can find one, they often OC by 1Ghz) and Nvidia GeForce 6800 GS (don't know about ATI's).