Have the same machine, with everything pretty much maxed out. What version of Windows are you on, how much RAM do you have currently and what graphics card? What do you use the machine for?
A CPU upgrade to an Intel Xeon W3580 (quad core @ 3.33 GHz, i7-975 equivalent) is cheap, about $35 on ebay. But, if you're on Vista (no longer updated) or Windows 7 (updates end in less than a year - January 14, 2020), it may not be worth it.
And I can attest that a SSD in the 435MT (I'd get at least 240 GB) makes a big difference as Tesla1856 mentioned. $30 - $50 for a new or used one.