The key in beating the OEM is build a high-end PC. OEM PC run at stock(exclude boutique PC builder). The majority of the cost is in the CPU(higher the frequency the more it cost). OEM doesn't OC the cpu, so you buy one on the same class at lower speed and OC. OEM will rape you on high end GPU-always cheaper going somewhere else. RAM upgrade cost is a joke with OEM. The biggest drawback for OEM is the MB-no OCing features. The PSU is bare minimum-if you going to add anything else later; going to have to upgrade the PSU.
DIY PC
1.better MB(base on your research) but cost you more
2.more ram for less cost
3.better GPU less cost
4. faster CPU for same cost
5.better PSU but cost you more(will have to replace anyways)
6. no crapware clean up
CONs:
1. warranty on 1 whole system instead of each part
2.The extra are cheap w/ oem(keyboard, mouse, speaker)
3. The OS is bundle with the PC(extra cost for you)
Overall, just hardware alone, the DIY cost less and better. (the better you OC the CPU, the more you save).