Let say this way, official Dell moderators and non Dell supporters like SpeedStep would provide valuable information within official parameters and we, users really appreciate for their help. However, when users tweak their PCs beyond official Dell recommendations please do not expect any confirmation from them in questions of upgrading processors beyond official max level and any overclocking of cheap Dells. In cases like this only us, users can confirm any upgrades beyond Dell's limits, so if someone confirms "The GX620 MT does support Core 2 Duo 6300" this could be very true. I just can say that mine Inspiron 570 CPU upgraded beyond Dell official max and overclocked!
Coming back to original question, if you provide your BIOS date I will check which CPU might work hypothetically.