If you're not afraid to build, which it seems you're not looking at a bare-bones system, here's a rig I was thinking about putting together for myself to replace my 10 year old Dell which is not my main rig.
You'd have to use your OS, keyboard, mouse, monitor and speakers/headset from your old machine.
As to savings:
You could drop the floppy drive (I just like to have one just in case) and the soundcard to save a few bucks. You could also skip the lightscribe DVD drive and save another couple of dollars. Finally, there are some decent cases that could save you another $10-20.
As to performance improvements:
For only a few dollars more you could get a much larger hard drive (I didn't need it as I have others I was going to use for back-up and storage drives). You could also really bump up the video card for another $10-20.
All parts quoted from newegg:
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Update11-119-106 COOLER MASTER Centurion 534 RC-534-KKN2-GP Black Aluminum & Mesh bezel / SECC Chassis ATX Mid Tower Computer Case - Retail
Model #:RC-534-KKN2-GP
Item #:N82E16811119106
Save Cancel $54.99 -$5.00 Instant $49.99
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Update13-128-357 GIGABYTE GA-G31M-ES2L LGA 775 Intel G31 Micro ATX Intel Motherboard - Retail
Model #:GA-G31M-ES2L
Item #:N82E16813128357
Save Cancel $52.99 $52.99
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Update14-130-394 EVGA 256-P2-N761-TR GeForce 8600 GTS 256MB 128-bit GDDR3 PCI Express x16 HDCP Ready SLI Supported Video Card - Retail
Model #:256-P2-N761-TR
Item #:N82E16814130394
Mail in Rebate
Save Cancel $54.99 $54.99
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Update17-182-075 Rosewill Stallion Series RD450-2-SB 450W ATX V2.2 Power Supply - Retail
Model #:RD450-2-SB
Item #:N82E16817182075
Save Cancel $49.99 -$10.00 Instant $39.99
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Update19-116-072 Intel Pentium E5200 Wolfdale 2.5GHz LGA 775 65W Dual-Core Processor Model BX80571E5200 - Retail
Model #:BX80571E5200
Item #:N82E16819116072
Save Cancel $69.99 $69.99
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Update20-134-046 Kingston 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300) Dual Channel Kit Desktop Memory Model KVR667D2N5K2/2G - Retail
Model #:KVR667D2N5K2/2G
Item #:N82E16820134046
Save Cancel $22.99 $22.99
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Update21-103-203 SAMSUNG Black 1.44MB 3.5" Internal Floppy Drive Windows 98SE/ ME/ 2000/ XP - OEM
Model #:SFD321B/LFBL1
Item #:N82E16821103203
Return Policy:30 Day Return Policy
Save Cancel $7.49 $7.49
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Update22-136-075 Western Digital Caviar SE WD1600AAJS 160GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #:WD1600AAJS
Item #:N82E16822136075
Save Cancel $41.99 $41.99
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Update27-135-186 ASUS 22X DVD Burner with LightScribe Black SATA Model DRW-22B1LT - Retail
Model #:DRW-22B1LT
Item #:N82E16827135186
Save Cancel $28.99 $28.99
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Creative Sound Blaster Audigy SE 7.1 Channels PCI Interface Sound Card - Retail
Model #:70SB057000000
Item #:N82E16829102002
Save Cancel $29.99 $29.99
Subtotal: $399.40