Neither of those cards is considered a real gaming card - they are more for watching videos and general surfing and office applications.
Here is a review on the HD 5450:
http://www.tomshardware.com/reviews/radeon-hd-5450,2549.html
I did not see any credible reviews of the GT 420 - probably because the sites that normally review cards do so for gaming purposes and this was not worth considering - to them as a gaming card.
You can find some good information about graphics cards at upgradevideocards.com:
http://www.upgradevideocards.com/index.html
Particularly note the page on non-gaming cards which lists some info on the HD 5450. The other pages should help you better understand the info in the article above, such as the explanations about frames per second comparisons, on this page:
http://www.upgradevideocards.com/gaming.html
The rest of the site includes info you might find helpful as well as links and guides to using them to find out more about good video cards. Another good reference is this site's monthly listing of the best video cards at different budget levels:
http://www.tomshardware.com/reviews/best-graphics-card,2964.html
Note the quick reference table on the back that groups roughly comparable cards and ranks the groups:
http://www.tomshardware.com/reviews/best-graphics-card,2964-7.html
I did not find the GT 420 on the list but the HD 5450 is. Note that most of the cards below it are discontinued models, some discontinued many many years ago.
If you get either of those HP systems, you will need to upgrade the video card to game. A key qualification for upgrading is that they have a strong enough power supply unit (PSU) to handle a more powerful card - often systems sold with only low end video cards don't have a PSU strong enough for much of an upgrade. You should check the PSUs on thos systems as well.