Demon Kagerou
I ended up going with a MSI Radeon HD 6670 1GB... seems like the highest I can go with my limited power supply of 300w and is within the $100ish budget. Model actually recommends 400w... >.>
It does seem to meet what I was going for in making games more playable. Also went from 4gb of ram to 10gb... feels pretty slick now!
Even a 6670 is a huge increase over that 4200. Those wattage recommendations aren't very useful however. What you need is sufficient 12V rail amperage. The only reason they list a total power supply wattage is because it is assumed that any power supply of that capacity should offer enough 12V amperage to support the card (in addition to everything else) but this isn't necessarily the case. Regardless, you definitely don't have a high end power supply. But the Radeon 7750 consumes approximately the same amount of power as the 6670. Either way, big improvement. You were trying to play it safe - nothing wrong with that.
I have to say that the additional memory really would not have done much of anything. Unless you're dabbling in rendering or VMs or something 4GBs of RAM is more than sufficient. More won't hurt. But any perceived impact is exactly that - perception, and not reality.
Windows Vista/7/8 will see a UI improvement with better graphics hardware though. So that card could be the reason if the UI feels more responsive now.