Theres no doubt that the i5 3570 is the fastest one, specially compared to your current CPU.
Between the 3570 vs 3450 there should be a slightly increase in performance from the 3570 (after all it has higher base and boost clock), in some games it may be the diference between decent/playable vs horrible.
Also to get the best performance if you can make sure the "new" second hand system have at least 2x4GB DDR3 memory sticks, for dual channel memory then the better (for the i5 3xxx, DDR3 1600MHz is very welcome).
But keep in mind that dual channel configuration is more important than the DDR3 speed.
As to wiat for a better CPU, theres no need to wait, there are tons of better CPU already in the market, but I think I understand what you meant, if you wana wait for having the budget for a better CPU, well thats something only you can answer. How bad/soon do you need the higher performance?
In this second hand market, any Core i7 3xxx/4xxx/6xxx will be a better bet, cause they had HT enable (so you go from a 4 cores 4 threads core i5, to a 4 cores / 8 threads core i7) thats a huge step in performance for all the tasks that you listed.
I know this is not what youre asking for but just to give you an idea, for a brand new system heres an example of what I would do:
PCPartPicker Part List
CPU: AMD Ryzen 5 2600 3.4 GHz 6-Core Processor ($125.98 @ Newegg)
Motherboard: MSI B450M PRO-VDH MAX Micro ATX AM4 Motherboard ($79.99 @ B&H)
Memory: Corsair Vengeance LPX 8 GB (2 x 4 GB) DDR4-3000 Memory ($47.99 @ Best Buy)
Storage: Western Digital Caviar Blue 1 TB 3.5" 7200RPM Internal Hard Drive ($44.99 @ Adorama)
Case: Cooler Master MasterBox Q300L MicroATX Mini Tower Case ($44.99 @ Best Buy)
Power Supply: EVGA BQ 500 W 80+ Bronze Certified Semi-modular ATX Power Supply ($65.98 @ Newegg)
Total: $409.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-04-03 08:50 EDT-0400
You can make the brand new system cheaper by going with a Ryzen 5 1600AF (usually sells on Amazon for less than U$90 (
https://www.amazon.com/AMD-Processo...1&keywords=ryzen+1600af&qid=1585918430&sr=8-1), is a rebranded R5 2600, same CPU, just a really tinny perofrmance diference 1~2% at the most.
Also swap the PSU for a cheaper model (not modular or semi modular like this:
https://pcpartpicker.com/product/xD...nze-certified-atx-power-supply-100-br-0450-k1).
You could skip the HDD and save $45, if you have one already.
The rest of the components are the less expensive and decent ones I could find.
I hope it helps,
Cheers!