Build the PC backward from what you need/want to do then you can get exactly what you need. Your PC is just getting to the point where some games will not work on it and your HD sounds like it will die any moment. Understand the specs required for the highest end game you need focusing on the video card, RAM and operating system. Once you know what you need you can determine what you can afford and build it from there. You may be able to get a PC at a good price with Vista and good RAM etc, but buy a separate video card. Sometimes it can work better than building from scratch (unless you have a lot more cash to put down on a high-end gaming machine.
Thanks i think i got what im looking for.
HD: Western Digital Caviar SE WD1600AAJS 160GB 7200 RPM SATA 3.0Gb/s Hard Drive
Ram: G.SKILL 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Dual Channel Kit Desktop Memory Model F1-3200PHU2-2GBNS
Graphics Card: SAPPHIRE 100251SR Radeon HD 4870 X2 2GB 512-bit (256-bit x 2) GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
Processor: AMD Phenom II X4 920 2.8GHz Socket AM2+ 125W Quad-Core Processor Model HDX920XCGIBOX
MotherBoard: ASUS M3A78-T AM2+/AM2 AMD 790GX HDMI ATX AMD Motherboard