I think i've mentioned what my choice. U could put another 4GB RAM in the future, it won't cost too much (but not guarantee the low price in your country,usually in US it costs $40-60, it's clear that u'r not in the US/Canada,probably in Europe, there isn't such Dell 3251 model in North America).
Personally I don't think 7670M that bad (GT 630M < HD 7670M < GT 640M). If I were u, I would rather choose i3-3110m/i5-3210m+HD 7670M than i3/i5-3xxxU, it has low CPU freq and bad cooling IMO. If CPU too low, it WILL bottleneck the GPU.
This page also indicates that HD 7670M can handle most games in medium settings.http
/www.notebookcheck.net/AMD-Radeon-HD-7670M.69483.0.html