Dimension 4500/4550 upgrade options

Max Processor for Dimension 4500, 4550 =
Intel Pentium 4 HT  3.06Ghz  533Mhz FSB 512k Cache ('Nothwood' 130nm) but NOT Intel P4 any Ghz 800Mhz FSB 1MB Cache ('Prescott' 90nm)
No HT HyperThreading for DIM 4500, as BIOS does not cater for it (Max BIOS Rev.A04 - for DIM 4550 Rev. A06, for HT to work (NOT A08))

For RAM being maxed out at 1GB, no - you can go to 2GB (But no further) See:
Dimension 4550: Maximum memory 2GB, not 1GB

Applies to Dimension 4500 also.

Is it worth it? Definitely, parts are cheap ( around £15 each for P4 3.06 and 2x1Gb memory)  and you have a fast and responsive computer instead of a sluggish one (Vital you have fastest HDD - WD5000AAKB (PATA) - no longer available new, so used wiil be ok). Other things you can do to optimise this 2002/2003 Technology:
For decent broadband and networking - Gigabit Card + Router (eg. NetGear) if your ISP is supplying decent MB/s

Install WINDOWS 7 (WIN 8 no go) and or Linux Ubuntu (Keep XP, but support ends April 2014)

Even though have Dell Precision 490 2xQuad Xeon (heavy on the juice), for daily use it's the Dimension 4550 with occasionally the Dimension 4500 to keep it updated and lubed.  All 3 PC's are maxed out..