you dont need paticularly high spec ,for what you use the PC for , so a medium spec machine with a 64 bit processor and 4 gb or more memory will perform the tasks you now do at around 4 or 5 times the speed . it cost big bucks to get hihg spec machines for the smallest of increase in speed . your older programs are unlikely to work with 64 bit but there are free one around (do a google search ) that will probably surprise you .
since 2004 I have been biulding all my own computers from scratch myself and have biult dozens for friends , prices are pretty low at the moment on hardware , the most expensive single item would be the Operating system .
if your machine is 10 years old you really need to move on , it will pack in when you least expect it and you will lose everything you have stored ,new machines can not only work much faster and more efficiently but hard drives are as cheap as they have ever been ,I recently installed a 1tb Sata 300 hard drive that cost just £62 . thats a huge amount of storage space .