Evo's are great value, I never noticed much difference if any during normal desktop day to day use between the Evo and Pro, my kids have the Evo in an old LGA 775 kit and it's still a huge upgrade.
I think as has been said, there is very little between many in day to day use, most differences in benchmarks being barely noticeable to the average user at home. Reliability is probably better than ever as we don't see a huge amount of complaints even with regard to budget SSD's when reading various website after sale reviews.
I take it it's an old system with that old drive, Serial ATA-300 spec yes? Sure I have 2x500 and 2x1tb of those still in the loft, the HDD of choice for the budget concious before SSD.