I have no experience with UBid, so I won't comment on them. Looking at the two units from Newegg, the second one (ThinkCenter M57) with an added single slot, low-profile graphics card should be plenty for your needs, but I do have some reservations.
Under no circumstance should you purchase the ThinkCenter M52 you linked. Single core and an AGP graphics card slot.
The ThinkCenter M57 is the better choice (dual-core), but my concern would be the location of the hard drive in relation to where the graphics card would be installed. Add in a low-profile ready card with a cooling fan to at least get some air movement underneath the hard drive.
-Wolf sends