The Gateway DX4200-09 is based on the OLD Phenom I processor, they are pretty slow.
The HP Pavilion p6210f is based on the new Athlon II which is a cut down Phenom II.
The Athlon II and Phenom II are near enough the same speed, the HP Pavilion p6210f has the faster quadcore.
The Pavilion also has 6 GBs of memory which is more attractive to the than the Gateway's 4 GBs.
Bare in mind the HP Pavilion p6210f is will make good multimedia rig, photoshop, encoding, Microsoft office etc. – (you’d need a dedicated gaming card if you are a gamer)
Sidenote, I find it strange that HP are building AMD rigs with 6 GBs and not 4 GBs or 8 GBs for dual channel!