As nna2 said, it will be a MASSIVE downgrade from what you have. I cannot stress this enough: IT. WILL. BE. A. MASSIVE. DOWNGRADE.
The E5300 is nearly, and in some cases more than, TWICE as fast as any Pentium 4 (which is the processor you have).
For more info, the processor you mention is a Pentium 4 550 3.4 GHz with Hyper-threading (http://ark.intel.com/products/27469/Intel-Pentium-4-Processor-550-supporting-HT-Technology-(1M-Cache-3_40-GHz-800-MHz-FSB))
However, that processor is 8 years old! The E5300 is much newer and much faster. Don't believe us? Check this out. This chart compares the E5300 to an 3.46 version of the Pentium 4 (http://www.anandtech.com/bench/Product/98?vs=93)